MSA

A MSA is a mail submission agent which is a program used by a computer to receive messages from the mail user agent (MUA). The MSA will contact another email server's mail transfer agent (MTA) to send the email to the next server where the recipient may have their mailbox or a server that relays the email to a server containing the recipient's mailbox.

RFC 2476 discusses the mail submission agent and reserves port 587 for MSA use.