Within the Internet email system, a message transfer agent ( MTA),MTA=Message Transfer Agent (similar to X.400 name) is found, e.g., in RFC 1506, RFC 2476, RFC 3461, RFC 3464, RFC 3865, RFC 3888, RFC 6409, RFC 5598. or mail transfer agent,MTA=Mail Transfer Agent (similar to Mail Transfer Protocol) is found, e.g., in RFC 2298, RFC 2305, RFC 3804, RFC 3798, RFC 4496, RFC 5442, RFC 5429. or mail relay is software that transfers electronic mail messages from one computer to another using the Simple Mail Transfer Protocol. RFC 5598, Internet Mail Architecture, D. Crocker (July 2009). In some contexts the alternative names mail server, mail exchanger, and MX host can be used to describe an MTA.
Messages exchanged across networks are passed between mail servers, including any attached data files (such as images, multimedia or documents). These servers also often keep mailboxes for email. Access to this email by end users is typically either via webmail or an email client.
An MTA works in the background, while the user usually interacts directly with a mail user agent. 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;See table at Email client#Port numbers 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). Upon final delivery, the field is added to the envelope to record the return path.
Common protocols for this are:
Submission of new email from a mail client is via SMTP, typically on port 587 or 465, and is now generally restricted to servers the user has an account with-such as their ISP. This is for policy, not technical, reasons so that providers have some means of holding their users accountable for the generation of spam and other forms of email abuse.