What is an MDA?
A Mail Delivery Agent (MDA) is the email server component that performs the last step of delivery. After the receiving MTA (Mail Transfer Agent) accepts a message over SMTP, it passes the message to the MDA, which writes it into the correct user's mailbox. The MDA handles local delivery and mailbox storage; it doesn't send mail across the internet.
What an MDA does
- Local delivery. Works out which mailbox the message belongs to and stores it, for example in mbox or Maildir format on a Linux mail server.
- Filtering and sorting. Applies server-side rules, such as moving newsletters into a folder or filing messages a spam filter has flagged into a Junk folder.
- Handing off to other programs. Some MDAs can pass a message through a script or scanner before storing it.
The MTA often hands messages to the MDA with LMTP (Local Mail Transfer Protocol), a simplified relative of SMTP made for this job. Well-known MDAs include Procmail, maildrop and the delivery agent built into the Dovecot mail server. In Microsoft Exchange Server, delivery into mailboxes is handled by Exchange's own transport services.
Where the MDA fits
- The sender's MUA (Mail User Agent) submits the message to its mail server.
- The sender's MTA finds the recipient domain's MX record in DNS and relays the message over SMTP.
- The recipient's MTA accepts it and passes it to the MDA.
- The MDA stores it in the recipient's mailbox.
- The recipient's MUA retrieves it with IMAP or POP3.
IMAP vs. POP3 retrieval
| IMAP | POP3 | |
|---|---|---|
| Ports | 143, or 993 with TLS | 110, or 995 with TLS |
| Where mail lives | Stays on the server; folders and read status sync across devices | Usually downloaded to one device and removed from the server |
| Best for | A phone, laptop and webmail used together | A single computer, or limited server storage |
For the CompTIA A+ exam
Core 1 (220-1201) objective 2.3 lists mail servers as a server role, and objective 2.1 covers the mail ports: SMTP 25, POP3 110 and IMAP 143. MDA isn't on the Core 1 acronym list, but knowing the chain (MUA to MTA to MDA, then IMAP or POP3 back to the MUA) helps you place each protocol: SMTP moves mail between servers, and IMAP and POP3 fetch it from the mailbox. MDA here has nothing to do with Model-Driven Architecture, a software design term.