Message transfer agent

Within the Internet email system, a message transfer agent (MTA),^[1] mail transfer agent,^[2] or mail relay is software that transfers electronic mail messages from one computer to another using the Simple Mail Transfer Protocol.^[3] In some contexts, the alternative names mail server, mail exchanger, or MX host are 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 often keep mailboxes for email. Access to this email by end users is typically either by webmail or an email client.