Page "Message transfer agent" Paragraph 3

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 < tt > Received </ tt > 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.