Editing Message transfer agent (section)

==Operation==
A message transfer agent receives mail from either another MTA, a [[mail submission agent]] (MSA), or a [[mail user agent]] (MUA). The transmission details are specified by the [[Simple Mail Transfer Protocol]] (SMTP). When a recipient mailbox of a message is not hosted locally, the message is relayed, that is, forwarded to another MTA. Every time an MTA receives an email message, it adds a {{mono|Received}} trace header field to the top of the header of the message,<ref>See [[Email#Message header]] for the format of an email message. Many MUAs allow users to see the raw ''message source'' directly, thereby allowing header inspection.</ref> thereby building a sequential record of MTAs handling the message. The process of choosing a target MTA for the next hop is also described in SMTP, but can usually be overridden by configuring the MTA software with specific routes.

[[File:E-mail.svg]]

An MTA works in the background, while the user usually interacts directly with a mail user agent. One may distinguish initial submission as first passing through an MSA—port 465 (or, for legacy reasons, optionally port 587) is used for communication between an MUA and an MSA, while port 25 is used for communication between MTAs, or from an MSA to an MTA.<ref>See table at [[Email client#Port numbers]]</ref> this distinction is clarified in {{IETF RFC| 8314}}.

For recipients hosted locally, the final delivery of email to a recipient mailbox is the task of a [[message delivery agent]] (MDA). For this purpose the MTA transfers the message to the message handling service component of the message delivery agent (MDA). Upon final delivery, the {{mono|Return-Path}} field is added to the envelope to record the [[return path]].