 One may distinguish initial submission as first passing through an MSA—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 first made in RFC 2476.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).