MDA

What it means

RFC 5598 calls the transfer of responsibility into the recipient's mailbox delivery, and places it in the MDA. The MDA knows the destination address in detail, may redirect a message to another address the recipient chose, and performs whatever delivery actions the recipient has specified, such as filing rules. It records the envelope sender in a Return-Path header and may add a Received header; the mail program then reads the stored message over IMAP, POP or a provider interface.

RFC 5598 notes that delivery can mean either this hand-over or the first time a person sees the message, and offers a simple test for the mail-system sense: whether a DSN can be generated. A message the MDA files into a spam folder has been delivered in that sense.