What is a Message Transfer Agent?
MTA, short for Message Transfer Agent, is a piece of software that is responsible for transferring emails between computers. MTA’s are essentially mail servers that are known by quite a few different names. It is referred to as a mail transport agent, mail transfer agent, mail relay, mail router, or even internet mailer.
Technically speaking, a message transfer agent (MTA) is a program used within a message handling system (MHS) for transmitting emails between the sender and receiver device or computer. The basic function of an MTA is the transmission of mail between users. Here the main support for an MTA is the exchange system with the server architecture that aids the transfer.
Some common examples of MTA are Microsoft Exchange, Exim, Sendmail, Amazon SES, and Oracle Beehive. In the corresponding sections, we will learn more about how the messaging system works and the role of MTA in the process. We will also touch upon the different types of MTA and the main functions they exhibit.
How does an MTA work?
As mentioned earlier, an MTA is a component of the message handling system. It works in conjunction with the other components to enable the email delivery process.
The main elements in the email delivery process are as follows:
1. Mail User Agent (MUA) - MUA is the first point of contact in this process. An MUA is usually an application used by users to send or receive emails.
2. Mail Submission Agent (MSA) - It is an intermediate element that receives mail from MUA and transfers it to MTA. Practically, most MTAs work as MSA’s to perform their functions.
3. Message Transfer Agent (MTA) - A MTA in the process can receive mail from either an MSA or another MTA, or even an MUA.
4. Message Delivery Agent (MDA) - This is the last stop where mails arrive before being sent into the users’ inbox.
So here, MTAs receive mail from an MSA, which, in turn, receives the mail from an MUA. Once the mail is received by an MTA the relaying process follows. If the recipient is not hosted locally the mail is forwarded to other MTAs. Finally, the mail is sent to the MDA which then delivers it into the recipient’s inbox.
The SMTP (Simple Mail Transfer Protocol) is used to send emails between servers (MTA) - also known as an SMTP relay. Then the message is routed directly over POP3 (Post Office Protocol - a one-way client-server protocol) or IMAP4 protocols. The IMAP and POP3 protocols are used by email clients to retrieve messages directly from the server.
Main functions of a Message Transfer Agent
Now that we know how MTA works in the email delivery process, let’s discuss the main functions of MTA:
- Accept emails sent from the MUA (mail user agent)
- Select a mail server to transfer emails, depending on the MX records and the domain name of the recipient.
- Process deferrals if any and track the delivery status.
- Send an auto-response to the sender ID, if the delivery fails.
Types of MTA servers
The two main types of MTA servers are classified based on where they are hosted. These types are on-premise MTA servers and cloud-based MTA servers. Some mail servers can be physical servers located at an organization’s premises. Other mail servers can be third-party virtually hosted servers that organizations can access to meet their email transfer needs. We will read about them in detail below.
1. On-Premise MTA servers
On-premise MTA servers are mostly hosted by large enterprises or companies that have an in-house email infrastructure that includes both the software and the hardware component. Here the mail servers use the organization’s servers where all transferred emails are saved in an indexed database.
The basic functionality of MTA remains the same in both cases. But building an on-premise MTA infrastructure can be expensive, although the goal here is to give the company complete control over its email transfer system.
2. Cloud-Based SMPT server
Cloud-based SMPT servers are, as the name suggests, a cloud-based infrastructure that enables email transfer. Examples of cloud-based SMPT servers are services like SendGrid or even Mailgun. The catch, in this case, is that, although the cloud-based servers are inexpensive they don’t allow complete control over the mail sending and delivery infrastructure. It is essentially a third-party system that organizations can make use of.
Examples of commonly used MTAs
Below we have listed some popular mail servers that are most commonly used. They come in both free and paid versions that use the SMPT protocol.
- Amazon SES- it’s a cloud-based mail server that is scalable and can be deployed by developers for several functions like mail transportation, mass communication, and marketing.
- Microsoft Exchange Server- this is a mail and calendaring server that operates exclusively on the Windows Server OS.
- Oracle Beehive- this is a paid collaboration platform that combines email, team collaboration, instant messaging, and much more.
Other notable and popular mail servers are EXIM, Halon MTA, Postfix, Sendmail, and Qmail.