You may have heard several names for a mail server, including mail relay, mail router, and Internet mailer. Among them, MTA is one of the most commonly used terms for a mail server. MTA may refer to a mail transfer agent, a message transfer agent, or a mail transport agent. That’s a lot of names all in use. And although the terminology can be tricky, MTA is important to understand. So, you might be wondering - what is an mta? In this article we will explain exactly how MTAs work, what they mean for email deliverability, sender protection and credibility plus much more.

What is MTA?

Mail / message transfer agents (MTAs) are a vital part of the Internet message handling system. MTAs transfer electronic mail messages between sender and recipient computers using the SMTP protocol, ensuring efficient message transmission.

MTA is responsible for many tasks in the email delivery process, including:

  • Queuing: Storing emails in a queue (or line-up) for sequential dispatch to the recipient's server.
  • Throttling: Regulating the email transmission rate to prevent overwhelming the recipient's server and potentially getting marked as spam.
  • Optimal Scheduling: Determining the best timing for email dispatch considering recipient time zones and other relevant factors.
  • Connection Oversight: Managing the initiation, upkeep, and termination of network connections essential for email transmission.
  • Data Transmission: Relocating email content, attachments, etc., between network points. Processing Deferrals: Handling temporary delivery issues, like recipient server congestion or downtime.
  • Generating bounces: Generating and transmitting bounce messages to the sender for undelivered emails.
  • Tracking delivery status: Monitoring and logging the delivery progress of each email—whether delivered, queued, bounced, or deferred.

How do MTAs work?

When an email is sent, a sequence of email delivery processes initiates.

An MTA acts as an intermediary in the email delivery process, receiving messages from Mail Submission Agents (MSAs) after their origin in Mail User Agents (MUAs), commonly known as email clients. Once an MTA receives an email, it engages in relaying. This is why MTAs are often called mail relays. If the recipient's server is on a local server, the MTA delivers the email to the Mail Delivery Agent (MDA). If the recipient isn’t hosted locally, MTA forwards messages to other MTAs, eventually delivering the email to the Mail Delivery Agent (MDA). The email is eventually delivered to the recipient's mailbox after being received by MDA. Email distribution is accomplished via SMTP or extended SMTP and POP3 or IMAP4.

Mail Queueing and Delivery

MTAs primarily employ the SMTP (Simple Mail Transfer Protocol) for sending emails and may use extensions like ESMTP (Extended SMTP) for enhanced functionalities. They operate using a store-and-forward model, queueing outgoing mail until delivery confirmation or expiration of predefined time limits.

After being delivered by the MTA, the mail enters a queue, anticipating the recipient server's response. Should it remain undelivered within the stipulated timeframe, the email gets rerouted to the email server. The M.T.A. recurrently attempts delivery, and if unsuccessful within the designated period, the mail is sent back to the mail client.

MTAs Impact on Email Deliverability

Mail Transfer Agents are central to email deliverability by managing sender credibility, filtering spam, authenticating emails, optimizing delivery routes, and handling delivery errors.

Email delivery may get affected by sending server performance, user authentication and the email content. The most crucial factor is the credibility of the website and the Default gateway to which the message is sent from.

Any indication of sender unreliability by recipient web servers may divert all emails from that source to the spam folder or result in bounce-backs. Mail transfer servers (MTAs), therefore, are crucial for maintaining sender reputation and ensuring successful email transmission.

MTAs contribute to the safeguarding and building of sender reputation by upholding sender trustworthiness, adhering to industry standards, and overseeing optimal transmission paths. Here's how MTAs can contribute to protecting and increasing your sending credibility:

Warm-up New IP Address

If you’re establishing your internet identity or initiating a new IP address for email sending, your virgin IP address should be “warmed-up” before being used at full load. That’s because with no email sending history, ISPs (Internet Search Providers) won’t recognise “cold” IP addresses and thus are more likely to be suspicious of large volumes of emails coming from them.

MTAs aid in this process by routing test emails during IP warming. This process helps you hit your daily targets and gradually increase your sending capacity. Using cold IP addresses in a controlled way is yet another solution. This process involves employing minimum bounds to filter emails to colder IPs, aligning the MTA with these limits.

Mailslurp can assist you in gradually warming up your cold IP address through the generation of disposable inboxes. By using these inboxes, you can simulate email sending scenarios, gradually increasing sending volume and establishing a positive sending reputation without risking your primary email infrastructure. Alternatively, just use Mailslurp’s inboxes, which are all pre-warmed.

Set up mail-sending workflows

Limits on incoming mail are established by each receiving domain independently. If they are surpassed, the receiving server may determine that the sending mail server is not reliable. To prevent this, adjust your Message Transport Agent (MTA) to cap sending volumes dynamically. If a domain rejects your emails, the MTA will temporarily halt the email queue. After a brief pause, it will resume sending at reduced rates to safeguard your domain and IP reputation.

Surpassing the "grey list"

Some email providers employ greylisting as a kind of preventive action. Greylisting serves as a filtering system, allowing legitimate senders easier passage than a blacklist. MTAs facilitate this process by organizing multiple queues and executing repeated delivery attempts for bounced emails. This proactive approach assists in surpassing the challenges posed by greylisting, ensuring more reliable email delivery.

Beyond these features, MTAs offer extensive other functionalities. These include configuring email throttling, establishing routing rules, monitoring outgoing mail flow, and plenty more.

Most used Mail Transfer Agents (MTAs)

  • Postfix: Known for its security features, simplicity, and strong performance.
  • Sendmail/Proofpoint: One of the oldest MTAs, powerful and widely used but can be complex to configure.
  • Exim: Highly configurable, flexible, and popular in many Unix-based systems.
  • Microsoft Exchange Server: Comprehensive suite with various email-related functionalities, widely used in Windows environments.
  • MailEnable: A Windows-based MTA known for its ease of use and robust features.
  • Qmail: Notable for its security features and simplicity, albeit with a smaller user base.
  • Courier Mail Server: Known for its flexibility and compatibility with various platforms.
  • Dovecot: Although primarily an IMAP/POP3 server, Dovecot can work in conjunction with other MTAs and is recognized for its speed and scalability.
  • OpenSMTPD: Favored for its simplicity and compatibility with OpenBSD systems.
  • Zimbra Collaboration Suite: Offers not only an MTA but also other collaboration tools like email, calendar, and document sharing.

How to select a MTA?

It's important to consider the following factors while considering an MTA:

  • Performance: which includes the speed, volume, and email sending capacity.
  • Configurability and migration ability
  • Vendor reputation and credibility
  • User-friendly pricing and infrastructure transparency
  • Security measures and data protection measures

Challenges in MTA Management

Managing MTAs poses multifaceted challenges for technical teams, encompassing performance optimization, reputation management, and adherence to email authentication standards like SPF, DKIM, and DMARC. Configuring and maintaining an MTA requires extensive expertise and resources.

Addressing Challenges with MailSlurp

This complexity in MTA management is where MailSlurp shines. By leveraging MailSlurp's services, technical teams can bypass the intricacies of MTA configuration and management, focusing instead on efficient email testing, scheduling and delivery.

Choosing the Right MTA Solution

Evaluating MTA solutions involves considering performance, configurability, vendor reputation, user support, and security measures.

On-Premise MTA vs. Cloud-Based SMTP Relay

While on-premise MTAs grant extensive control, they entail high costs, space requirements, and a time-consuming setup process. Conversely, cloud-based SMTP relays, like MailSlurp, offer ease of use, scalability, and cost-effectiveness, making them an attractive choice for moderate email sending needs.

Exploring On-Premise MTAs vs Cloud-Based Relay Services for Email Delivery

Mail Transfer Agents (MTAs) and Cloud-Based Relay Services are pivotal in email delivery, each with distinct advantages worth considering for your email strategy.

Mail Transfer Agents (MTAs)


  • Control: MTAs provide extensive control over email infrastructure, making them ideal for enterprises needing precise configurations and integrations.
  • Scalability: While setup may be time-consuming, MTAs offer scalability for high email volumes and customizations.
  • Integration: Seamlessly integrate with in-house tools and software, ensuring a cohesive email ecosystem.


  • Cost & Space: On-premise MTAs demand substantial investment, both in initial setup costs and physical space for hardware.
  • Complexity: Configuration and management require technical expertise, often necessitating dedicated personnel.

Cloud-Based Relay Services


  • Ease of Use: Quick setup and user-friendly interfaces make cloud-based relays accessible, ideal for startups and moderate email needs.
  • Cost-Efficiency: These services offer cost-effective solutions, requiring no infrastructure investment or space.
  • Scalability: Easily scale email sending volumes as needed without the complexities of hardware management.


  • Control & Customization: Users may have limited control over the infrastructure and configurations compared to on-premise MTAs.
  • Dependency: Reliance on external services may pose constraints in terms of control and specific integrations.

Choosing the Right Solution

MTAs suit enterprises seeking complete control, customizations, and robust integration capabilities. However, they require significant investments and technical know-how.

Cloud-Based Relay Services are perfect for startups and businesses with moderate needs, offering cost-effective solutions and easy scalability. While they lack some control, they provide accessible, hassle-free setups.

Ultimately, the choice between MTAs and Cloud-Based Relay Services depends on your organization's scale, requirements, and desired level of control over email infrastructure. Consider your goals and resources before making a decision for optimal email delivery.

Mailslurp’s Cloud-Based Relay Service

MailSlurp's SMTP relay service combines reliability, scalability, and ease of use. It offers dedicated IPs, actionable analytics, suppression lists, and configurable options to enhance email deliverability.

Leveraging MailSlurp for Enhanced Email Management

Developers can benefit from MailSlurp's features, such as in-depth analytics, domain-specific metrics, and seamless integration capabilities. By utilizing MailSlurp, users gain control over deliverability and a comprehensive solution for effective email management.


With any MTA, you can attain a high degree of deliverability. While some users make full use of the onsite solution's capabilities, others choose the simple plan if they don't want to employ all of its features. Hence make sure your MTA meets the requirements of the project.

For a powerful mail transfer solution try MailSlurp today.