Mail Agent

This chapter describes Mail Agents and their purpose in the email ecosystem.

   +-----+   +-----+   +------------+
   | MUA |-->| MSA |-->| Border MTA |
   +-----+   +-----+   +------------+
                             |
                             |
                             V
                        +----------+
                        | Internet |
                        +----------+
                             |
                             |
                             V
+------------------+   +------------+
| Intermediate MTA |<--| Border MTA |
+------------------+   +------------+
          |
          |
          V
       +-----+   +-----+
       | MDA |-->| MUA |
       +-----+   +-----+

Flow of emails in the mail ecosystem

MUA (Mail User Agent)

Client application that allows receiving and sending emails. It can be a desktop application such as Microsoft Outlook/Thunderbird/… or web-based such as Gmail/Hotmail/… (the latter is also called Webmail).

MSA (Mail Submission Agent)

The MSA is responsible for the incoming traffic in the mail system.

A server program that receives mail from an MUA, checks for any errors, and transfers it (via SMTP) to a MTA.

MTA (Mail Transfer Agent)

The MTA is responsible to deliver the mails to the recipients’s mail exchange, using the DNS logics.

A server application that receives mail from the MSA, or from another MTA. It will find (through name servers and the DNS) the MX record from the recipient domain’s DNS zone in order to know how to transfer the mail. It then transfers the mail (with SMTP) to another MTA (which is known as SMTP relaying) or, if the recipient’s server has been reached, to the MDA.

• A “border MTA” is an MTA that acts as a gateway between the general Internet and the users within an organizational boundary.
• A “delivery MTA” (or Mail Delivery Agent or MDA) is an MTA that actually enacts delivery of a message to a user’s inbox or other final delivery.
• An “intermediate MTA” is any MTA that is not a delivery MTA and is also not the first MTA to handle the message.

MDA (Mail Delivery Agent)

A server program that receives mail from the server’s MTA, and stores it into the mailbox. MDA is also known as LDA (Local Delivery Agent).

An example is Dovecot, which is mainly a POP3 and IMAP server allowing an MUA to retrieve mail, but also includes an MDA which takes mail from an MTA and delivers it to the server’s mailbox.