mail transfer agent (X.400) (MTA)
The term Mail Transfer Agent(MTA) is used synonymously with Message Transfer Agent (MTA). It is a mail server that receives messages from a Mail User Agent( MUA) or from another mail server and makes them available for forwarding using Simple Mail Transfer Protocol( SMTP).
The actual data transport to the recipient or another mail server is handled by the Mail Delivery Agent( MDA).
In this software program for switching e-mails, each Mail Transfer Agent attempts to reach the receiving mail server directly via the domain name. The DNS server is then used to determine the IP address of the mailbox host.
MHS systems were already described in the first RFCs in the 1980s. So was the implementation of the Mail Transfer Agent (MTA) and the Simple Mail Transfer Protocol (SMTP), which were first described in RFC 821.