Mail server

A mail server (hereinafter, the Server) is any digital construct that is located in a computer network under its distinguishable hostname in order to accept, analyze, adjust, clarify, and transfer electronic mail messages (or, simply, emails) from mail user agents (MUAs) to mail exchangers (MX hosts) and vice versa.

The Server can also refer to mail server software. More broadly, email software may refer to all the software utilized for MUAs, the Servers, or mail exchangers.

Features

To communicate with:

Mail exchangers, all the Servers use the Simple Mail Transfer Protocol (SMTP).
Mail user agents (MUAs), the Servers use a variety of protocols, most commonly, POP3 and/or IMAP.

With regard to:

Outgoing messages, the Servers may be designed to:
1. Accept messages from mail user agents.
2. Analyze messages looking for their inconsistencies and/or missing data.
3. Adjust messages while correcting inconsistencies and/or adding the missing data.
4. Transfer messages to mail exchangers.
Incoming messages, the Servers may be designed to:
1. Accept messages from mail exchangers.
2. Analyze messages looking for their inconsistencies and/or missing data.
3. Add information about the found inconsistencies and/or missing data to the messages.
4. Transfer messages to mail user agents.

The Servers communicate with MUAs and mail exchangers:

MUAs are hosted at either:
- End-user devices such as a mail app on a cell phone; or
- Mail service provider such as Gmail locations.
Mail exchangers are hosted by Internet service providers (ISPs), who also run DNS resolvers, which provide mail exchangers with DNS records.

So, the Servers are located between those two. Usually, they hosted by local computing devices that are connected to the Internet.

Mail submission agent (also known as outgoing mail server, message submission agent or by its acronym MSA), which is a computer application used to accept, analyze, correct, and send emails that end-users enter into their MUAs;
Mail transfer agent (also known as message transfer agent or by its acronym MTA), which is a computer application used to transfer emails between computers;
Mail delivery agent (also known as incoming mail server, message delivery agent or by its acronym MDA), which is a computer application used to deliver an email to its recipient's MUA.