What is a mail server (MTA)?

A mail server (or MTA: Mail Transfer Agent) is software that runs on an Internet computer.

The mail server receives its messages from a Mail User Agent (MUA) and sends them via the Simple Mail Transfer Protocol (SMTP).

The Domain Name System (DNS) is necessary for this, because the sending mail server reaches the receiving mail server via its domain name. For this purpose, it uses a DNS server to determine its IP address.

See also:
Wikipedia: Mail Transfer Agent