Message delivery agent

A message delivery agent (MDA), or mail delivery agent, is a computer software component that is responsible for the delivery of e-mail 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]

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.

Unix

On Unix-like systems, procmail and maildrop are the most popular MDAs. The Local Mail Transfer Protocol (LMTP) is a protocol that is frequently implemented by network-aware MDAs.^{[citation needed]}

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

References

^ RFC 5598, Internet Mail Architecture, D. Crocker (July 2009)
^ 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.

[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.

[1]

[2]