What is a Mail User Agent?

A mail agent is a computer software component that delivers email to the mailboxes of local recipients. It is also called Local Delivery Agent (LDA).

Many mail processing software products bundle multiple mail delivery agents with mail transfer agent components, providing users with the convenience of customizing the specifics of mail delivery.

On Unix-like systems, procmail and maildrop are the most popular MDAs. The Local Mail Transfer Protocol (LMTP) is a protocol often implemented through network-aware MDAs. [Citation needed]

The mail delivery agent is usually not started from the command line, but is usually called by the mail delivery subsystem, such as a mail delivery agent or a mail retrieval agent.

List of MDA software for Unix-like platforms.

Cyrus IMAP-mail server suite with mail delivery agents;

dovecot-mail server suite with mail delivery agents;

fdm (email utility)-a combined mail retrieval agent and mail delivery agent-for complex mail ordering, this man page references user maildrop;

fetchmail-mainly mail retrieval agent (MRA);

getmail-simpler and more secure alternative to modern fetchmail;

mpop-faster and more modern fetchmail alternative;

mailagent-An MDA that manually configures many parameters, such as time zone; there seems to be at least one Debian package.

maildrop or courier-maildrop (same procedure?)-traditional procmail replacement;

postdrop or postfix-maildrop (same procedure?)-a mail delivery agent for use with the postfix mail transfer agent; see also postfix daemons smtp, local, virtual.

sieve-A more modern procmail alternative in the GNU Mailutils package

Outdated MDA software for Unix-like platforms.

bin / mail, the MDA part of Sendmail-Sendmail is one of the oldest email packages.

DBMail-MDA written in C by Eelco van Beek and Roel Rozendaal. Provides IMAP and POP3 daemons using MySQL or PostgreSQL mail storage. old? This URL looks like a web service.

dystill-Filters MySQL database-driven mail delivery agents-outdated or outdated, no Debian package.

procmail-Obsolete because it is not maintained; old, but still used.

When sending a mail through a PDA, the size of the mail sent by a PDA generally does not exceed the size of a PC and other PDAs due to the limitation of the storage size of the PDA itself. Therefore, the PDA does not consider sending mail through the mail proxy, but the PDA directly interacts with the mail server through the SMTP protocol.

When receiving mail through a PDA, since most of the mail sent by a PC exceeds the range that the PDA can receive, it receives mail through a mail agent. At this time, the interaction between the mail proxy and the PDA is as follows: The PDA will first initiate a connection to port 110 of the mail proxy server. Since the mail proxy has been listening to port 110, when it finds that there is a connection on port 110, it pretends to be a client connection. The POP3 mail server sends a "pseudo" response message "+ OK PoP3 server ready" to the PDA to "deceive" the PDA and continue to issue a formal service request. After the PDA sends the user name and password, the mail proxy server initiates a connection to the mail server instead of the PDA according to the user name and password. When the mail server sends a real response message "+ OK POP3 server ready" to the mail proxy, The line between the mail agent and the PDA is established. Each time the PDA sends a request to the mail server, the mail agent instead of the PDA initiates a request to the mail server, and the server's response results are processed accordingly according to the user's profile before being delivered to the PDA user.