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2025

Delivery

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. 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.

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.

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