What is Mail Server in Linux?
A Mail Server, also known as an Email Server, is an application that is capable of storing and transferring an email. When an email is sent, it passes through various mail servers before it reaches its recipient. Although mails are almost instantly delivered nowadays, it is a very complicated process. There are plenty of mail server applications that run on Linux operating systems.
What are the best Linux Mail Servers?
Let’s have a look at the 7 best Mail Servers for Linux server distros like CentOS, Ubuntu Server, AlmaLinux, Rocky Linux which we can trust with our mails.
Exim
Exim was developed as a message transfer agent at the University of Cambridge for use on Unix systems connected to the Internet. It is possible to install Exim instead of Sendmail, despite its configuration being a little bit different. It gets frequent updates for bug fixing. It has excellent customization and configuration capabilities. On the security side, it comes with three log mechanisms named as the main log, panic log, reject log. In addition to these features, it supports MySQL, PostgreSQL, SQLite, Oracle DB, and Redis databases, and even more.
- Download Exim Mail Server: exim.org/mirrors.html
- Exim Documentation: exim.org/docs.html
Postfix
Postfix that was founded by a Google software engineer, Wietse Venema runs not only Linux also AIX, BSD, HP-UX, IRIX, MacOS X, Solaris, Tru64 UNIX, and other UNIX systems. And it supports MySQL, Memcache, SQLite, PostgreSQL, LDAP, CDB, and Berkeley Database as data storage. Postfix can be defined as an effective open-source Linux mail server with its features as junk mail control, database support, and convenient log management.
It comes with a content filter, access control mechanism, virtual domain support, configurable status notifications. It can deliver over 300 emails per second from a single Postfix instance. By providing defense mechanisms against malware and spambots, it offers a secure service.
- Download Postfix Mail Server: postfix.org/download.html
- Postfix Documentation: postfix.org/documentation.html
SquirrelMail
SquirrelMail that is a standards-based webmail package written in PHP includes built-in pure PHP support for the IMAP and SMTP protocols. Moreover, it comes with all pages rendered in pure HTML 4.0 for maximum compatibility across browsers. SquirrelMail has some benefits like easy configuration and installation, strong MIME support, address books, and folder manipulation. As SquirrelMail is licensed under the GNU General Public License (GPL), it is free to use.
- Download SquirrelMail Mail Server: squirrelmail.org/download.php
- SquirrelMail Documentation: squirrelmail.org/documentation/
Axigen
Axigen has innovative features including an integrated mailing list server, malware, and spam protection mechanisms. This mail server software runs on Linux, Windows, or Docker. It has local and remote access, can easily synchronize mobile. On the security side, it comes with an extensive security toolset. It is available for many Linux distributions but you have to pay after use free trial full-featured for 60 days.
- Download Axigen Mail Server: axigen.com/mail-server/download/
- Axigen Documentation: axigen.com/documentation/
Citadel
One-year-old mail server software Citadel is an open-source software developed for Linux and BSD Unix distributions but supports most systems. It comes with a pack of features include e-mail, calendar, address books, bulletin boards, mailing list server, instant messaging, and more. It has its own implementation of standard server protocols like SMTP, IMAP, Sieve, POP3, XMPP. For the storage of all contents, the Berkeley DB database is preferred. OpenLDAP and Microsoft Active Directory are supported by Citadel.
- Download Citadel Mail Server: citadel.org/download.html
- Citadel Documentation: citadel.org/documentation.html
Zimbra Email Server
Open-source software Zimbra Collaboration Suite (ZCS) is an efficient mail server for Linux users. It transfers messages efficiently and takes less time to migrate with its powerful migration tools. It also comes with additional features such as group calendars, and document sharing with flexible tooltips, drag-and-drop items, and convenient UI menus. It has also a mobile web client application and is ready for using the private and public cloud as well as a closed-source enterprise edition. For strengthening security, Zimbra provides a real-time backup feature.
- Download Zimbra Email Server: zimbra.com/downloads/
- Zimbra Documentation: zimbra.com/documentation/
Horde
Horde Groupware Webmail Edition enables users to read, send and organize email messages and manage and share calendars, contacts, tasks, notes, files, and bookmarks with the standards-compliant components from the Horde Project. Horde Groupware Webmail Edition has many features including IMAP and POP3 webmail client, 3-pane and 3-column-layouts, message filtering, HTML message composition with WYSIWIG editor, shared IMAP folders, and more.
Download Horde Mail Server: horde.org/download/webmail
Horde Documentation: horde.org/apps/webmail/docs/INSTALL
What are the types of Mail Servers?
There are two types of mail servers depending on their functions:
- Incoming mail servers
- Outgoing mail servers
Outgoing Mail Server
An outgoing mail server is an application that handles the delivery process of an email. Outgoing mail servers use SMTP protocol and transfer the email from the client to an incoming mail server.
What is SMTP?
SMTP or Simple Mail Transfer Protocol is an internet communication protocol designed for email deliveries. It is a part of the application layer of the TCP/IP protocol and works with Mail Transfer Agent (MTA) to deliver emails across networks.
Incoming Mail Server
An incoming mail server is an application that stores emails and sends them to the inbox. It can also be referred to as POP (Post Office Protocol), POP3, or IMAP (Internet Message Access Protocol) server.
What is POP3?
POP3 is the third version of the popular email protocol, used in incoming mail servers. POP3 allows users to download email messages on the local computer. POP3 works on 110 and 995 ports by default.
What is IMAP?
The Internet Message Access Protocol or IMAP is also a very common email protocol used for retrieving emails. It accesses emails on a server from a client by using 143 and 993 ports.
What is the difference between POP3 and IMAP?
POP3 assumes that the emails are accessed by only one application. However, IMAP allows multiple clients to access the emails simultaneously, which makes it a better option for users that need to access the emails from different locations and emails are managed by multiple users.
How does a Mail Server work?
There are many different mail servers that can run on a Linux server. The common working steps of a mail server is as follows:
- When the sender hits the Send button, the sender’s email client contacts the SMTP server delivers the content to the SMTP server.
- The SMTP server checks the sender’s email address and domain. If the receiver’s domain and the sender’s domain is the same, the mail is directly sent to the sender’s POP3 or IMAP server.
- If the sender’s and receiver’s domains are different, the mail server uses the IP address found by the DNS query to contact the receiver’s SMTP server. Then the receiver’s SMTP server analyzes the incoming mail and transfers it to the POP3 or IMAP server.
- Finally, the mail is downloaded by the receiver’s mail client and this completes the delivery process.