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 trace header field to the top of the header of the message, 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. 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. this distinction is clarified in . 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 field is added to the envelope to record the
return path. ==Transfer versus access==