MDA (Mail Delivery Agent)

An MDA (Mail Delivery Agent) is the final server-side step in receiving email. It takes a message the MTA has accepted for a local user and writes it to that user's mailbox, often after running filtering rules.

Why it matters

This is the stage where mail is sorted into folders, rejected for quota reasons or flagged as spam. Dovecot's LDA/LMTP and procmail are common MDAs. Failures here look different from routing failures: the message was accepted by the server, but still ends up in the wrong place or is bounced later with a full-mailbox error. Separating the MDA from the MTA also allows one transport layer to serve many storage backends.

Example

A server accepts a message for jane@example.com. The MDA then:

1. Checks Jane's Sieve rule: if the subject contains [ALERT], file it in the Alerts folder.
2. Checks her quota; if it is over the limit, it returns a temporary failure like 452 4.2.2 Mailbox full.
3. Otherwise it stores the message in her mailbox (for example in Maildir format).

Common mistakes

• Assuming that an accepted SMTP message is guaranteed to reach the inbox; the MDA can still divert it.
• Ignoring mailbox quotas, which cause bounces for mail that was otherwise valid.
• Writing server-side filters that loop or silently discard mail.

Related

• MTA
• MUA
• Mail Server
• Email Bounce

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