Have you ever wondered how an email makes its journey to your inbox? What happens behind the scenes? Does your email simply travel from Point A to Point B, or are there multiple steps in between ensuring it reaches the correct recipient? Despite its apparent simplicity, email delivery has a fascinating backstory. Numerous components work behind the scenes to deliver your emails. Today, let's delve into one of the primary components: the MTA, or Mail Transfer Agent.

What is an MTA (Mail Transfer Agent)?

A mail transfer agent (MTA), sometimes known as a mail server, mail router, internet mailer, mail relay, etc., is a software application that receives emails from senders and then forwards them for delivery. It is responsible for routing emails across the Internet—hence the name. An MTA uses protocols such as SMTP (Simple Mail Transfer Protocol) to send emails and IMAP (Internet Message Access Protocol) or POP3 (Post Office Protocol) to retrieve emails from a server.

Let’s understand MTA with the help of an example:

Imagine you're sending a physical letter through the postal service.

You (the sender): You're analogous to someone composing an email and hitting send.

The Postal Service (MTA): The MTA is like the postal service. It's the intermediary responsible for transferring your letter from your local post office to the recipient's local post office.

Local Post Office (Email Client): Your local post office is like your email client (such as Gmail, Outlook, etc.). This is where you drop off your letter (compose your email) and receive incoming letters (emails).

Recipient's Local Post Office (Recipient's Email Server): The recipient's local post office represents the email server of the person you're sending the email to. This is where your letter (email) is eventually delivered for the recipient to pick up.

Recipient (the receiver): The recipient receives your physical letter or email.

MTA (Mail Transfer Agent) is the critical middleman that ensures your letter (email) is properly routed and delivered to the correct destination, just as the postal service ensures your physical letter reaches its intended recipient.

You will understand MTA better once you know how an email is sent or received.

The Critical Process of Sending Emails

The process of sending or receiving emails may seem complex, but it is actually pretty simple.

The following diagram explains the basic process of how emails are sent and received.

1) Initiation by User: Users compose emails using their email client (Gmail, Outlook, etc.).

2) Connection to the Sender's Email Server (MTA): The email client connects to the sender's Email Server (MTA), which prepares the email for sending.

3) Sender's Verification: The sender's Email Server verifies the sender's credentials and checks for any issues associated with the sender's domain or IP address.

4) Recipient Address Resolution: The sender's Email Server checks if the recipient's domain matches the sender's. If not, it uses the Domain Name System (DNS) to find the recipient's Email Server.

5) Connection with the Recipient's Email Server: The sender's Email Server connects to the recipient's Email Server using the SMTP protocol.

6) Email Transmission: The email is transmitted from the sender's Email Server to the recipient's Email Server over the SMTP connection. An intermediate relay server (SMTP relay) may be used if the recipient's server is unreachable.

7) Delivery to Recipient's Mailbox: The recipient's Email Server delivers the email to the recipient's mailbox using the Message Delivery Agent (MDA).

Four players (MUA, MSA, MTA, MDA) play a crucial role in the email delivery process. To sum up their role, I have explained them below in short:

Mail User Agent (MUA):

The Mail User Agent (MUA), also known as an email client, is the interface users use to compose, send, receive, and manage email messages. Examples of MUAs include web-based clients like Gmail and Outlook and desktop clients like Thunderbird and Apple Mail. MUAs interact directly with users, providing an interface for composing and managing their email accounts.

Mail Submission Agent (MSA):

The Mail Submission Agent accepts outgoing email messages from MUAs and submits them for delivery to the appropriate Mail Transfer Agent (MTA). The MSA communicates with MUAs using protocols like SMTP (Simple Mail Transfer Protocol) to receive emails and ensure they are correctly formatted and ready for delivery. It then forwards these messages to the MTA for further processing.

Message Transfer Agent (MTA):

The Message Transfer Agent is responsible for routing email messages from the sender's mail server to the recipient's mail server. MTAs use protocols like SMTP to transfer messages between servers and determine the most efficient path for delivery. They handle tasks such as message queuing, routing, relaying, and delivery, ensuring that messages are transmitted accurately and securely across the Internet.

Mail Delivery Agent (MDA):

The Mail Delivery Agent (MDA) delivers incoming email messages from the mail server to the recipient's mailbox. MDAs receive messages from the MTA and deposit them into the appropriate user's mailbox for retrieval by their Mail User Agent (MUA). MDAs may perform additional tasks such as filtering, sorting, and mailbox management to organize and manage incoming messages effectively.

Understanding MTA’s ‘Store-and-forward’ Model

MTA uses the "store-and-forward" model to exchange emails.

In this model, the sender's MTA first receives an email, which examines the recipient's address to determine the next course of action.

If the recipient's MTA is inaccessible or offline, the sender's MTA temporarily stores the email message in its queue, ensuring it's held until it can be successfully delivered.

The sender's MTA periodically attempts to forward the stored message to the recipient's MTA, persisting until either the recipient's MTA acknowledges receipt or a predefined timeout occurs.

Once the recipient's MTA becomes available, it retrieves the email from its queue and delivers it to the recipient's mailbox for access through its email client.

This "store-and-forward" mechanism offers reliability, as messages are safeguarded even if either party is temporarily offline. It also efficiently manages email traffic at various paces and resilience against network disruptions, ensuring email messages are reliably delivered across networks.

Main Functions of MTA

The main functions of a Mail Transfer Agent (MTA) include:

a) Message Routing

MTAs are responsible for routing email messages from the sender's mail server to the recipient's mail server. This involves determining the optimal path for message delivery across the Internet based on the recipient's email address and domain.

b) Message Queuing

MTAs queue outgoing messages for delivery if the recipient's mail server is temporarily unreachable or delayed. Messages are held in the queue until they can be successfully delivered.

c) Message Delivery

Once a recipient's mail server is reachable, the MTA delivers the queued messages to the recipient's server. This involves connecting to the recipient's server and transmitting the message data.

d) Protocol Handling

MTAs support various email protocols, such as SMTP (Simple Mail Transfer Protocol), for sending and receiving email messages. They may also support additional protocols, such as POP3 (Post Office Protocol version 3) and IMAP (Internet Message Access Protocol), for retrieving emails from a server.

e) Spam and Virus Filtering

Many MTAs include built-in spam and virus filtering capabilities to protect users from unwanted or malicious email messages. These filters analyze incoming messages and apply rules to identify and block spam and malware.

f) Authentication

MTAs may implement authentication mechanisms, such as SMTP authentication, to verify users' identities and prevent unauthorized access to the mail server. This helps ensure that only legitimate users can send and receive email through the server.

g) Logging and Monitoring

MTAs generate logs to track the flow of email messages through the system. These logs include information such as message delivery status, timestamps, sender and recipient details, and any errors encountered during processing. Monitoring tools may also track server performance and detect potential issues.

h) Relaying

MTAs may act as relays to forward email messages between different domains or networks. This allows messages to be routed through intermediate servers to reach their final destination, particularly when direct delivery is impossible.

Popular MTAs You Can Explore

1. Sendmail (Proofpoint)

Sendmail, now part of Proofpoint, is one of the oldest and most widely used Mail Transfer Agents (MTAs), particularly in Unix-based systems. It is known for its robustness, flexibility, and extensive configuration options. For decades, Sendmail has been a foundational component of email infrastructure, offering features such as mail queuing, routing, and delivery. Proofpoint's enhancements to Sendmail focus on adding advanced security features to protect against email threats, making it a comprehensive email delivery and security solution.

Advantages:

• Highly configurable and customizable.
• Long-standing reputation for reliability and stability.
• Extensive support for complex email routing and delivery scenarios.
• Enhanced security features, such as protection against spam, phishing, and malware.

2. Postfix

Postfix is a popular open-source Mail Transfer Agent known for its security, speed, and ease of configuration. It is designed to be modular and extensible, making it suitable for various email delivery scenarios. Postfix is widely used in small and large email systems, offering features such as SMTP server support, built-in spam filtering, and support for multiple delivery transports.

Advantages:

• Secure and robust architecture.
• Easy to configure and maintain.
• Excellent performance and scalability.
• Built-in support for spam filtering and virus scanning.

3. Exim

Exim is another open-source Mail Transfer Agent known for its flexibility, reliability, and ease of configuration. It is prevalent in Unix and Linux environments, offering extensive routing capabilities, support for multiple message transports, and integration with various authentication mechanisms.

Advantages:

• Highly customizable and flexible configuration options.
• Support for advanced routing and delivery scenarios.
• Efficient resource utilization and low overhead.
• Comprehensive logging and debugging capabilities.

4. Qmail

Qmail is an open-source Mail Transfer Agent developed by Daniel J. Bernstein. It is known for its security, simplicity, and efficiency. Qmail's design minimizes vulnerabilities and ensures reliability, making it a popular choice for environments prioritizing security and stability.

Advantages:

• Security-focused design with a strong emphasis on minimizing vulnerabilities.
• Simple and efficient architecture.
• Built-in support for email forwarding and aliasing.
• Reliable and stable performance.

5. Axigen

Axigen is a commercial Mail Transfer Agent and email server solution designed for businesses of all sizes. It offers a comprehensive suite of email services, including SMTP, IMAP, POP3, and webmail access, as well as advanced collaboration features such as calendaring and contact management.

Advantages:

• All-in-one email server solution with support for multiple protocols.
• Easy to deploy and manage, with a user-friendly web-based administration interface.
• Scalable architecture that can accommodate growing email needs.
• Robust security features, including antivirus and antispam protection.

6. Mutt - Command Line Email Client

Mutt is a powerful and flexible command-line email client for Unix-like operating systems. It is highly customizable and scriptable, making it popular among power users and those who prefer a text-based interface for email management.

Advantages:

• Lightweight and fast, ideal for low-resource environments.
• Highly customizable, with support for scripting and keyboard shortcuts.
• Wide range of features, including support for multiple email accounts, threading, and MIME attachments.
• Integration with external tools and utilities for enhanced functionality.

7. OpenSMTPD

OpenSMTPD is a free and open-source Mail Transfer Agent that comes pre-installed with many Unix-like operating systems, including OpenBSD. It is designed to be secure, simple, and easy to configure, making it a popular choice for small to medium-sized email deployments.

Advantages:

• Simple and intuitive configuration syntax.
• Built-in support for modern email protocols, including SMTP, LMTP, and ESMTP.
• Focus on security and reliability, with features such as privilege separation and privilege revocation.
• Active development community and regular updates.

8. Postal

Postal is a fast, lightweight, open-source Mail Transfer Agent designed for high-performance email delivery. It is optimized for efficiently delivering large volumes of email, making it suitable for use cases such as transactional email delivery and email marketing campaigns.

Advantages:

• High-performance architecture optimized for delivering large volumes of email.
• Lightweight and resource-efficient, with minimal overhead.
• Support features such as IP throttling, bounce processing, and delivery tracking.
• Easy to deploy and manage, with a simple configuration interface.

9. Microsoft Exchange

Microsoft Exchange Server is a widely used enterprise-level Mail Transfer Agent and email server solution developed by Microsoft. It offers comprehensive email and collaboration services, including email hosting, calendaring, contacts management, and integration with other Microsoft Office applications.

Advantages:

• Feature-rich email and collaboration platform with support for email, calendaring, contacts, tasks, and notes.
• Tight integration with other Microsoft Office applications, such as Outlook, SharePoint, and Teams.
• Robust security and compliance features, including built-in protection against spam, malware, and data loss.
• Scalable architecture that can accommodate the needs of small businesses and large enterprises.

10. Amazon SES (Simple Email Service)

Amazon SES is a cloud-based Mail Transfer Agent that Amazon Web Services (AWS) provides. It is designed to help businesses and developers send transactional and marketing emails quickly and efficiently, with high deliverability rates.

Advantages:

• Scalable and cost-effective email-sending solution with pay-as-you-go pricing.
• High deliverability rates and reputation management to ensure emails reach recipients' inboxes.
• Integration with other AWS services, such as Amazon S3, Amazon Lambda, and Amazon CloudWatch.
• Simple API and SMTP interface for easy integration with applications and systems.

On-premises Vs. Cloud-based Mail Servers

On-premise Mail Servers

On-premise mail servers are hosted and managed within an organization’s data center. Because they are physically located on your premises, you have more control over your email infrastructure's hardware, software, configuration, and security.

You must also purchase and maintain your hardware, software licenses, and infrastructure setup. If you need more control over your email environment, go for on-premise mail servers. Examples of on-premises mail server software include Microsoft Exchange Server, Zimbra, IBM Domino (formerly Lotus Domino), and others.

Cloud-based Mail Servers

Cloud-based mail servers are hosted and managed by third-party cloud service providers in their data centers. You can access these over the internet on a subscription or pay-as-you-go basis.

They provide more flexibility and scalability without requiring you to manage the physical hardware or infrastructure.

These solutions typically offer webmail access, collaboration tools, security features, and automated backups.

Examples of cloud-based mail server providers include Microsoft Exchange Online, Google Workspace (formerly G Suite), and Amazon WorkMail.

So, what’s the difference between on-premise and cloud-based mail servers

The difference between the servers can be understood under the following heads:

a) Location

Cloud-based: Hosted and managed off-premises in third-party data centers.

On-premises: Hosted and managed locally within the organization's infrastructure.

b) Control

Cloud-based: Limited control over hardware and infrastructure configuration; managed by the service provider.

On-premises: Complete control over hardware, software, configuration, and security.

c) Costs

Cloud-based: Typically operates on a subscription or pay-as-you-go model, with lower upfront costs but ongoing operational expenses.

On-premises: Higher upfront costs for hardware, software licenses, and infrastructure setup, along with ongoing maintenance costs.

d) Scalability

Cloud-based: Offers scalability with the ability to quickly scale resources up or down based on demand.

On-premises: Scaling typically requires additional hardware purchases, configuration changes, and resource allocation.

e) Maintenance and Updates

Cloud-based: Managed and maintained by the service provider, including security patches, updates, and backups.

On-premises: Requires an in-house IT team to manage and maintain the entire email infrastructure, including updates, backups, and troubleshooting.

The choice between cloud-based and on-premises mail servers depends on your organization's needs, preferences, budget, and IT capabilities.

The Difference Between an MTA and an SMTP

We have already covered the MTA and its crucial role in the email delivery process. SMTP is the fundamental protocol these mail servers use to exchange and route email messages across the Internet. While talking about SMTP, it is also important to briefly understand SMTP relay. It refers to forwarding an email message from one SMTP server to another. This is often necessary when the original SMTP server cannot deliver the message directly to the recipient's server, such as when the recipient's server is inaccessible or when the sender's server is not configured to deliver emails directly to the recipient's domain.

In simpler terms, SMTP is the protocol for sending emails. SMTP relay is a mechanism to route emails between different SMTP servers to ensure delivery to the intended recipients.

SMTP defines the standards and procedures for transferring email data between mail servers, specifying how messages should be formatted, transmitted, and received.

It operates on port 25 by default and utilizes a client-server architecture, enabling mail servers to communicate with each other and exchange email messages. When an MTA sends an email message to another MTA, it establishes an SMTP connection. It follows the SMTP protocol to transmit the message, ensuring it reaches its intended destination securely and efficiently.

Let’s cover the differences between MTA and SMTP in more detail:

Role and Functionality

MTA: The MTA is a software application that routes email messages between mail servers and manages queuing routing and delivery tasks.

SMTP: SMTP is a communication protocol used by mail servers to exchange email messages and relay them to their final destinations, defining the rules and procedures for email transmission.

Scope of Operation

MTA: The MTA encompasses a broader set of functionalities, including message queuing, routing decisions, and delivery management.

SMTP: SMTP focuses specifically on the protocol used for email transmission, outlining the standards and procedures for exchanging email data between mail servers.

Implementation and Usage

MTA: MTAs are software applications implemented on mail servers to handle the routing and delivery of email messages.

SMTP: SMTP is a protocol implemented within MTAs and used by mail servers to communicate with each other, facilitating the exchange of email messages.

Interactions with Other Components

MTA: MTAs interact with various components in the email delivery process, including Mail User Agents (MUAs), Mail Submission Agents (MSAs), and Mail Delivery Agents (MDAs), to ensure seamless email transmission.

SMTP: SMTP defines the communication protocol MTAs use to exchange email messages. It enables interoperability between different mail servers and facilitates the routing of email traffic across the Internet.

Leveraging MTAs For Email Marketing

MTAs play a crucial role in email marketing by providing different tools to engage your audience effectively. How you use MTA for your email marketing needs will depend on your requirements; however, below are the popular ways an MTA can help enhance your email marketing:

a) Executing Promotional Campaigns

You can utilize MTAs to send promotional emails announcing sales, discounts, or new product launches. MTAs can handle the bulk sending of promotional emails while ensuring compliance with email regulations and maintaining deliverability rates.

b) Sending Drip Campaigns

MTAs can support drip email campaigns, allowing you to send automated emails over time.

This strategy is effective for nurturing leads, onboarding new customers, and re-engaging inactive subscribers, helping businesses stay top-of-mind with their audience.

c) Tracking and Analytics

MTAs provide insights into email performance through tracking and analytics features. Businesses can monitor metrics such as open rates, click-through rates, and conversion rates to evaluate the effectiveness of their email marketing campaigns.

This data enables continuous optimization and refinement of marketing strategies to achieve better results.

d) Reliable Delivery

MTAs efficiently handle the transmission process to ensure reliable delivery of marketing emails to recipients. They manage bouncebacks, monitor email queues, and retry delivery attempts to maximize email deliverability rates.

e) Email Authentication

MTAs provide robust email authentication mechanisms such as SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting, and Conformance). These protocols ensure your emails are securely transmitted and protect against spoofing, phishing, and fraud. By leveraging MTAs for email authentication, you can establish trust with your audience and enhance deliverability rates.

f) Integration with CRM

MTAs seamlessly integrate with existing Customer Relationship Management (CRM) systems, enabling synchronization of contact lists, customer data, and email campaign metrics. Integrating MTAs with your CRM lets you personalize email communications based on customer information, segment your audience effectively, and track campaign performance. This integration streamlines workflows, improves targeting accuracy, and enhances the overall effectiveness of your email marketing strategy.

Impact of MTA on email deliverability

The Mail Transfer Agent (MTA) plays a significant role in email deliverability, affecting whether email messages reach recipients' inboxes or get filtered out as spam.

Here are several ways in which the MTA impacts email deliverability:

IP Reputation: The reputation of the IP address used by the MTA can significantly impact email deliverability. Email service providers (ESPs) and spam filters assess the reputation of the sending IP address to determine whether to accept, reject, or filter incoming email messages. MTAs must maintain a positive IP reputation by following best practices, such as sending relevant and engaging content, managing bounce rates, and avoiding spam traps.

Authentication Protocols: MTAs can implement authentication protocols like SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting, and Conformance) to verify the authenticity of email messages. Properly configured authentication protocols increase the likelihood of email delivery by establishing trust between the sending domain and the recipient's mail server.

Message Formatting: MTAs ensure email messages are correctly formatted according to industry standards and best practices. This includes adhering to MIME (Multipurpose Internet Mail Extensions) standards for encoding multimedia content and complying with HTML and CSS coding guidelines to optimize rendering across different email clients and devices.

Delivery Optimization: MTAs employ various techniques to optimize email delivery and minimize the likelihood of messages being flagged as spam. This includes managing message queues efficiently, implementing throttling mechanisms to control sending rates, and prioritizing delivery to reputable domains with strong email delivery policies.

Spam Filtering: MTAs incorporate spam filtering mechanisms to identify and block unsolicited or malicious email messages. These filters analyze incoming messages for characteristics commonly associated with spam, such as suspicious content, misleading subject lines, and excessive use of images or links. MTAs must balance effective spam filtering with minimizing false positives to ensure legitimate emails are delivered successfully.

Feedback Loops: MTAs can establish feedback loops with email service providers to receive notifications about user complaints and email delivery issues. These feedback loops enable senders to promptly identify and address potential deliverability problems, such as high complaint rates or spam trap hits, improving email deliverability and sender reputation.

How do you select the right MTA?

Selecting the right Mail Transfer Agent (MTA) ensures efficient and reliable email delivery within your organization or for your clients.

Here are steps to help you choose the correct MTA:

a) Assess Your Requirements

Identify your organization's specific email requirements, including the volume of email traffic, the number of users, security and compliance needs, integration with existing systems, scalability requirements, and budget constraints.

b) Research Available Options

Research the various MTAs available on the market. Consider open-source and commercial solutions like Postfix, Sendmail, Exim, and Microsoft Exchange Server. Evaluate their features, capabilities, compatibility with your infrastructure, and community support.

c) Evaluate Features and Functionality

Evaluate each MTA's features and functionality to determine whether they meet your organization's needs. Key features include message queuing and delivery, routing capabilities, support for authentication protocols (SPF, DKIM, DMARC), spam and virus filtering, logging and monitoring, and scalability.

d) Consider Security and Compliance

Assess each MTA's security and compliance features to ensure they meet your organization's requirements. Look for MTAs that offer robust security measures, such as encryption of email traffic, support for TLS (Transport Layer Security), and compliance with industry regulations such as GDPR, HIPAA, and PCI DSS.

e) Evaluate Performance and Reliability

Evaluate each MTA's performance and reliability to ensure it can effectively handle your organization's email traffic. Consider factors such as message throughput, latency, uptime, and fault tolerance. Look for MTAs with a proven track record of reliability and scalability in handling large volumes of email traffic.

f) Consider Integration and Compatibility

Consider how well each MTA integrates with your existing email infrastructure, including mail servers, directory services, email clients, and third-party applications. Then, choose a seamless MTA compatible with your organization's systems and tools.

g) Assess Support and Documentation

Evaluate the level of support and documentation provided by the MTA vendor or community. Look for MTAs with comprehensive documentation, user guides, tutorials, and community forums where you can seek assistance and troubleshoot issues. Consider the availability of technical support options, such as paid support contracts or community support channels.

h) Trial and Testing

Before making a final decision, consider conducting trials or testing each MTA in a test environment to evaluate its performance, compatibility, and ease of use. Pilot the selected MTAs with a small subset of users or email traffic to assess their suitability for your organization's needs.

i) Cost Considerations

Evaluate each MTA's total cost of ownership (TCO), including initial setup costs, licensing fees (for commercial solutions), ongoing maintenance, and support fees. Consider the scalability and flexibility of pricing models, such as per-user licensing or subscription-based pricing, to ensure they align with your budgetary constraints.

Conclusion

In summary, the Mail Transfer Agent (MTA) is a critical component of email infrastructure, facilitating the reliable and efficient transmission of email messages. By understanding the functions, components, and impact of MTAs, organizations can optimize their email delivery processes, enhance sender reputation, and ensure effective communication with their stakeholders.

Mail Transfer Agents (MTAs) FAQs

Q.1 What is a Mail Transfer Agent (MTA)?

A Mail Transfer Agent (MTA) is software that routes and delivers email messages between mail servers. It plays a crucial role in email delivery, ensuring that messages are transmitted securely and efficiently across networks.

Q.2 How does an MTA differ from an email client?

While email clients are involved in the communication process, they serve different purposes. An MTA operates on the server side, handling the routing and delivery of email messages. In contrast, an email client is a software application end-users use to compose, send, receive, and manage emails.

Q.3 What are some popular MTAs?

Some popular MTAs include Postfix, Sendmail, Exim, Microsoft Exchange Server, and Qmail. These MTAs vary in features, performance, and suitability for different use cases, but they all serve the fundamental purpose of routing and delivering email messages.

Q.4 What protocols do MTAs use for email transmission?

MTAs use the Simple Mail Transfer Protocol (SMTP) to transfer email messages between mail servers. However, they may also support other protocols, such as POP3 (Post Office Protocol version 3) and IMAP (Internet Message Access Protocol), for retrieving emails from mailboxes.

Q.4 How do MTAs handle email authentication and security?

MTAs implement various authentication mechanisms such as SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting, and Conformance) to verify the authenticity and integrity of email messages. These protocols help prevent email spoofing, phishing, and fraud, enhancing email security.

Q.5 What role do MTAs play in email deliverability?

MTAs play a crucial role in email deliverability by ensuring that email messages are routed and delivered to recipients' inboxes reliably and efficiently. They manage bouncebacks, monitor email queues, and comply with email regulations to maximize deliverability rates.

Q.6 Can MTAs be used for email marketing?

Yes, MTAs can be leveraged for email marketing campaigns. They facilitate bulk sending, personalization, segmentation, tracking, and analytics, enabling businesses to execute email marketing strategies and engage with their audience effectively.

Q.7 How do MTAs handle email queues and retries?

MTAs maintain email queues to manage the transmission of email messages. Suppose a message delivery fails initially (e.g., due to network issues or recipient server unavailability). In that case, MTAs will retry delivery according to predefined retry intervals until the message is successfully delivered or reaches a maximum retry threshold.