Email remains the killer information and communications technology. Email volume shows no sign of diminishing, despite the increasing popularity of collaborative messaging tools. There will be over 143 trillion emails sent in 2026.
Messages are exchanged between hosts using the Simple Mail Transfer Protocol with software programs called mail transfer agents, and delivered to a mail store by programs called mail delivery agents, frequently referred to as email clients.
Within the Internet email system, a message transfer agent, or mail transfer agent, or mail relay is software that transfers electronic mail messages from one computer to another using SMTP. The terms mail server, mail exchanger, and MX host are also used in some contexts.
Here’s our verdict on the finest email servers for Linux. All of the servers are published under an open source license.
Let’s explore the 21 email servers. For each title we have compiled its own portal page, a full description with an in-depth analysis of its features, together with links to relevant resources.
| Email Servers | |
|---|---|
| Exim | Highly configurable mail transfer agent |
| OpenSMTPD | Implementation of the server-side SMTP protocol with additional extensions |
| mox | Modern, secure, all-in-one email server |
| DBMail | Enables storing and retrieving mail messages from a database |
| WildDuck | Opinionated email server |
| Stalwart | All-in-one mail and collaboration server |
| Mailu | Self-hosted mail server stack |
| Mail-in-a-Box | Self-hosted email server solution |
| Postfix | Popular mail transfer agent |
| Maddy | Composable all-in-one mail server |
| Haraka | Modern SMTP server |
| Dovecot | Secure POP3 server that supports mbox and maildir mailboxes |
| Courier | Integrated mail/groupware server |
| Cyrus IMAP | Email, contacts and calendar server |
| Zimbra | Zimbra Collaboration Suite |
| Modoboa | Mail hosting and management platform |
| Citadel | Exchange-killer groupware server |
| chasquid | SMTP server with a focus on simplicity, security, and ease of operation |
| Sendmail | Mail transfer agent for sophisticated mail configurations |
| dma | DragonFly Mail Agent |
| DavMail | POP/IMAP/SMTP/Caldav/Carddav/LDAP exchange gateway |
This article has been updated to reflect the changes outlined in our recent announcement.
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In this list, I miss “Kolab”, which is a true fully Open Source Groupware (no restrictions) and has almost all professional features out of the box.
As you say, Kolab is groupware software. We cover groupware software separately here Best Free Groupware Software. We recommend Kolab as a groupware solution; it came 3rd.
Thanks for the reply. Yes you are absolutely right, however, Citadel and Zimbra are also groupware software, and seeing them in this list I thought of Kolab
That’s true, and that why we limited the number of groupware solutions that offer a good email server.
Opensmtpd is missing in this list. It is the standard smtp implementation on OpenBSD, but it is available on major Linux distros as well. Very lightweight to configure.
Ok
After migration to Debian 12, I replaced my Kolab Groupware with iRedMail (I also tested Modoboa).
iRedMail and Modoboa are both worth mentioning.
How are this servers graded?
What makes exim so much better than postfix, for example?
The individual pages provide no info or criteria for the scoring system
Please see this FAQ which covers your query.
In the past, we published multiple ratings charts rather than an overall rating but people complained the articles were too long. So we publish just the overall ratings chart. The other charts are still prepared but only supplied to people who help the site.