Message delivery agent

Short description: Computer software that delivers email to a mailbox

A message delivery agent (MDA) or mail delivery agent is a computer software component that is responsible for the delivery of email messages to a local recipient's mailbox.[1] It is also called a local delivery agent (LDA).

Within the Internet mail architecture, local message delivery is achieved through a process of handling messages from the message transfer agent, and storing mail into the recipient's environment (typically a mailbox).

Spam filtering usually occurs at the MDA under modern email architectures.[2]

## Contents

* 1 Implementation
* 1.1 Unix
* 1.1.1 Invocation
* 1.2 Unix-like
* 2 See also
* 3 References

## Implementation

Many mail handling software products bundle multiple message delivery agents with the message transfer agent component, providing for site customization of the specifics of mail delivery to a user.

A diagram of the flow of email messages between two participants, Alice and Bob.

### Unix

#### Invocation

The mail delivery agent is generally not started from the command line, but is usually invoked by mail delivery subsystems, such as a mail transport agent, or a mail retrieval agent.

### Unix-like

* Cyrus IMAP \- A mail server suite that includes a mail delivery agent
* dovecot \- A mail server suite that includes a mail delivery agent
* fetchmail \- Primarily a Mail retrieval agent (MRA)
* getmail \- simpler, more secure, modern fetchmail alternative
* fdm — modern replacement for both fetchmail and procmail from the author of tmux
* maildrop or courier-maildrop - traditional procmail replacement, part of Courier Mail Server, but can also be used with other mail servers
* procmail \- old, but still used
* bin/mail, the MDA part of Sendmail \- Sendmail is one of the oldest email packages
* Sieve mail filtering language \- a standardised mail filtering language; also, a modern replacement for procmail from the GNU Mailutils package

## See also

* Message transfer agent (MTA)
* Mail retrieval agent (MRA)
* Message submission agent (MSA)
* Message user agent (MUA) a.k.a. email client
* E-mail agent (infrastructure) (MxA)

## References

1. ↑ RFC 5598, Internet Mail Architecture, D. Crocker (July 2009)
2. ↑ Levine, John (June 10, 2025). "SMTP headers in DATA block?". mailsec (Mailing list). Internet Engineering Task Force. Retrieved June 10, 2025. "All my experience says that the right place to do mail filtering is the MDA. It has all of the information and has all the options about what to do with the mail. There are MUAs that try to do filtering but that's mostly an historical artifact from a long time ago when MDA filtering didn't happen."

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