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.
Contents
Features
Protocols
- 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.
Functions
- With regard to:
- Outgoing messages, the Servers may be designed to:
- Accept messages from email clients.
- Analyze messages looking for their inconsistencies and/or missing data.
- Adjust messages while correcting inconsistencies and/or adding the missing data.
- Transfer messages to mail exchangers.
- Incoming messages, the Servers may be designed to:
- Accept messages from mail exchangers.
- Analyze messages looking for their inconsistencies and/or missing data.
- 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.
- Transfer messages to email clients.
- Outgoing messages, the Servers may be designed to:
Hosting
- 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.
- Email clients are hosted at either:
Agents
- 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.
Best practices
- https://skrilnetz.net/setup-your-own-mailserver/
- https://www.linuxbabe.com/mail-server/block-email-spam-postfix
- https://manuals.gfi.com/en/kerio/connect/content/server-configuration/mail-delivery-and-dns-records/essential-dns-records-for-mail-delivery-and-spam-protection-1223.html
- https://www.cyberpunk.rs/mail-server-setup-postfix
Software
General comparison
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 6/8/2019 3/5/2019 2/10/2019 10/22/2018 06/29/2019 Introduction 2000 2002 1995 1996 1998 OS Linux/Unix Yes Yes Yes No Yes 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 Category Features Courier Dovecot Exim MS Exchange Postfix
Communication protocols
Communication protocols supported by the popular Clients Category Features Courier Dovecot Exim MS Exchange Postfix Transfer SMTP Yes No Yes Yes Yes Access IMAP Yes Yes via Dovecot, etc. Yes via Dovecot, etc. POP3 Yes Yes via Dovecot, etc. Yes via Dovecot, etc. Compatibility IPv6 Yes Yes Yes Yes Yes Category Features Courier Dovecot Exim MS Exchange Postfix
Security
Security features in the popular Clients Category Features Evolution Outlook RoundCube Thunderbird Zimbra SSL/TLS support Secure POP3 SSL/TSL SSL SSL/TSL SSL/TSL SSL/TSL Secure IMAP4 SSL/TSL SSL/TSL SSL/TSL SSL/TSL SSL/TSL Secure SMTP SSL/TSL SSL/TSL SSL/TSL SSL/TSL SSL/TSL Secure NNTP SSL/TSL Unknown No SSL/TSL SSL/TSL Secure LDAP SSL/TSL Unknown SSL/TSL SSL SSL/TSL SNI Unknown Unknown Unknown Yes Unknown OCSP No Unknown Unknown Yes Unknown CRL No Unknown Unknown Yes Unknown PGP support inline Yes No Unknown Yes Unknown PGP/MIME or OpenPGP Yes No Yes Yes No S/MIME support protocol Yes Yes Yes Yes Unknown OCSP Unknown Unknown Unknown Yes Unknown CRL Unknown Yes Unknown Unknown Unknown Certificates on tokens, smartcards support Unknown Yes Unknown Yes Unknown Category Features Evolution Outlook RoundCube Thunderbird Zimbra
