Local IPC??”Inter-process communications are often used instead of SMTP when transferring
a message between programs within a system.
Upon receiving a message, the MTA places it in a queue to be processed by an MDA (mail delivery
agent). Mail delivery agents come in two varieties:
Local MDAs??”Deliver messages to mailboxes on the local server. Simple versions copy
messages directly to a specified mailbox, while complex implementations can alter messages
or delivery parameters based on user-specified rules.
Remote MDAs??”Deliver messages over the network to remote servers. Full remote
MDAs use DNS (the Domain Name System) to determine the mail exchanger hosts for
recipient addresses and deliver to the best one available for each. Simple remote MDAs
(sometimes called ???null clients???) forward messages to a central server to continue the
delivery process. Most remote MDAs are capable of either method and will act as configured
by the administrator that performed the configuration.
You will often see the term MTA used in reference to the software that performs both
MTA and MDA functions. This is a carry-over from older designs that did not separate
the functions and is still fairly accurate given the fact that most mail server implementations include a
minimum of an MTA, remote MDA, and basic local MDA.
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