Mail submission agent

A mail submission agent stands between the MUA and the MTA in a mail system. It acts as a sort of "receptionist" for messages coming in to a mail system from MUAs. It does error checking and verification (such as verifying that hostnames are FQDNs, checking the legitimacy of local hostnames before appending the local domain portion, fixing headers, etc. etc.) before passing the message off to the MTA for delivery. It can also change the sender's e-mail address from a local user account to some type of standardized e-mail address such as lastname.firstname@localdomain.com.

MSAs are relatively new. Prior to their existence, all this work was handled by the MTA.