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 email clients 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 email clients, the Servers, or mail exchangers.

Features

To communicate with:

Mail exchangers, all the Servers use the Simple Mail Transfer Protocol (SMTP).
Email clients, 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 email clients.
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. This information may further be used by email clients to combat spam.
4. Transfer messages to email clients.

To communicate with email clients and mail exchangers, the Servers shall be located between those two.

Email clients are hosted at either:
- End-user devices such as a mail app on a cell phone; or
- Mail service provider such as Gmail locations.
The Servers are hosted by local computing devices that are connected to the Internet. On the one side, they can be colocated with email clients, especially if the email client is hosted by a mail service provider. On another side, the Server can can colocated with mail exchangers when the mail service provider is an Internet service provider (ISP). Unix-based operating systems include the Server in their bundles, so do some end-user applications such as MediaWiki, Moodle, and Redmine. At the same time, the Servers can also be hosted separately from email clients and mail exchangers.
Mail exchangers are hosted by Internet service providers (ISPs), who also run DNS resolvers, which provide mail exchangers with DNS records.

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 email clients;
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 email client.

General comparison of the actively-developed *Servers*
Category	Features	Courier	Dovecot	Exim	MS Exchange	Postfix
Legal properties	Ownership	OSS	OSS	OSS	Proprietary	OSS
	Developer	Sam Varshavchik	Timo Sirainen, contributors	Community	Microsoft	Wietse Venema, contributors
	License	GPL	MIT-L, LGPLv2	GPL	Unknown	IBM-PL, EPL
	Latest release	1.0.8	2.3.5	4.92	2019 RTM	3.4.6
History	Latest release	6/8/2019	3/5/2019	2/10/2019	10/22/2018	06/29/2019
History	Introduction	2000	2002	1995	1996	1998
OS	Linux/Unix	Yes	Yes	Yes	No	Yes
OS	Windows	No	No	No	Yes	No
Technology properties	Description	Full-stack mail server best known for its IMAP server component	IMAP and POP3 server	SMTP server	Full-stack mail and calendar server	SMTP server
	Database	No	Yes	Yes	via ESE	Yes
	File system	maildir	maildir, mbox, dbox	Own	Own	Own
	Webmail	Yes	No	No	Yes	No
Category	Features	Courier	Dovecot	Exim	MS Exchange	Postfix

Security features in the popular *Clients*
Category	Features	Courier	Dovecot	Exim	MS Exchange	Postfix
SSL/TLS support	SMTP over TLS	Yes	No	Yes	Yes	Yes
	POP over TLS	Yes	Yes	No	Yes	No
	SSL	Yes	Yes	Yes	Yes	Yes
Security features other than SSL/TLS	IMAP IDLE	Yes	Yes	via Dovecot, etc.	Yes	via Dovecot, etc.
Security features other than SSL/TLS	DANE
Filtering	Sieve
Category	Features	Courier	Dovecot	Exim	MS Exchange	Postfix