# Mail server

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A mail server consists of multiple components. A mail transfer agent (MTA) receives and sends emails via SMTP. Received and accepted emails are then passed to a mail delivery agent (MDA), which stores the mail in a mailbox (usually in mbox or Maildir format). If you want users to be able to remotely access their mail using email clients (MUA), you need to run a POP3 and/or IMAP server.

## Software

This article or section needs language, wiki syntax or style improvements. See Help:Style for reference.

Reason: The information would be better presented using a feature comparison table. (Discuss in "Software" Talk:Mail server#Classify everything under "Software")

This article or section needs expansion.

Reason: Add mailutils somewhere. (Discuss in talk:mail server#Should mailutils be added somewhere to mail server#software?)

All of these software include a mail delivery agent.

- dma — Tiny mail transport agent.

  https://github.com/corecode/dma || dmaAUR

- Exim — A highly configurable mail transfer agent.

  https://exim.org/ || exim

- OpenSMTPD — A mail transfer agent, part of the OpenBSD project.

  https://opensmtpd.org/ || opensmtpd

- Postfix — A mail transfer agent, meant to be fast, easy to administer, and secure.

  http://www.postfix.org/ || postfix

### POP3/IMAP servers

- Courier — A mail transfer agent, providing POP3, IMAP, webmail and mailing list services as individual components.

  https://www.courier-mta.org/ || courier-mtaAUR

- Cyrus IMAP — A mail transfer agent with a custom mail spool format, provides POP3 and IMAP services.

  https://www.cyrusimap.org/ || cyrus-imapdAUR

- Dovecot — An IMAP and POP3 server written to be secure, fast and simple to set up.

  https://dovecot.org/ || dovecot

- UW IMAP — An IMAP/POP server.

  https://web.archive.org/web/20191028114408/https://www.washington.edu/imap/ || imap

### Standalone MTAs

The factual accuracy of this article or section is disputed.

Reason: The classification of MTAs is ambiguous, the tools at the top (exim, postfix, opensmtpd) also provide a `sendmail` executable. (Discuss in Talk:Mail server#)

These packages all provide a `sendmail` executable with similar options.

- msmtp — A minimal mail transfer agent.

  https://marlam.de/msmtp/ || msmtp

- Sendmail — A deprecated mail transfer agent.

  https://www.sendmail.org/ || sendmailAUR

- sSMTP — An unmaintained, minimal mail transfer agent.

  || ssmtpAUR

### Standalone MDAs

- fdm — A simple program for delivering and filtering mail.

  https://github.com/nicm/fdm || fdm

- Procmail — A program for filtering, sorting and storing email (unmaintained).

  http://www.procmail.org/ || procmail

- Maildrop — A mail filter/mail delivery agent used by the Courier Mail Server.

  https://www.courier-mta.org/maildrop/ || courier-maildropAUR

See also Wikipedia:Comparison of e-mail servers.

## Ports

Purpose | Port | Protocol | Encryption
--- | --- | --- | ---
Accept mail from other MTAs. | 25 | SMTP | STARTTLS
Accept submissions from MUAs. | 587 | SMTP | STARTTLS
465 | SMTPS | implicit TLS
Let MUAs access mail. | 110 | POP3 | STARTTLS
995 | POP3S | implicit TLS
143 | IMAP | STARTTLS
993 | IMAPS | implicit TLS

Note: Implicit TLS is more secure than STARTTLS because the latter is vulnerable to man-in-the-middle attacks. For more information, see [1] and RFC:8314.

## MX record

Hosting a mail server requires a domain name with an MX record pointing to the domain name of your mail transfer agent. The domain name used as the value of the MX record must map to at least one address record (A, AAAA) and must not have a CNAME record to conform with RFC 2181, otherwise you may not get mail from some mail servers. Configuring DNS records is usually done from the configuration interface of your domain name registrar.

## TLS

Warning: If you deploy TLS, be sure to follow Server-side TLS to prevent vulnerabilities.

To obtain a certificate, see OpenSSL#Usage.

## Authentication

There are various email authentication techniques.

### Sender Policy Framework

From Wikipedia:

  Sender Policy Framework (SPF) is an email validation protocol designed to detect and block email spoofing by providing a mechanism to allow receiving mail exchangers to verify that incoming mail from a domain comes from an IP Address authorized by that domain's administrators.

To allow other mail exchangers to validate mails apparently sent from your domain, you need to set a DNS TXT record as explained in the Wikipedia article (there is also an online wizard). To validate incoming mail using SPF you need to configure your mail transfer agent to use a SPF implementation. There are several SPF implementations available: libspf2, perl-mail-spf and perl-mail-spf-query.

SPF validation support
Courier | Yes, built-in
Postfix | Yes
Sendmail | through Milter and spfmilter-acmeAUR
Exim | Yes, requires libspf2
OpenSMTPD | No
Cyrus IMAP | ?

The following websites let you validate your SPF record:

- SPF Record Checker
- SPF Email test

Tip: SPF can even be helpful for domains not used to send email. Publishing a policy like `v=spf1 -all` makes any mail server enforcing SPF reject emails from your domain name, thus preventing misuse.

### Sender Rewriting Scheme

The Sender Rewriting Scheme (SRS) is a secure scheme to allow forwardable bounces for server-side forwarded emails without breaking the Sender Policy Framework.

For Postfix, see Postfix#Sender Rewriting Scheme.

### DKIM

DomainKeys Identified Mail (DKIM) is a domain-level email authentication method designed to detect email spoofing.

Available DKIM implementations are OpenDKIM and perl-mail-dkim.

## Testing

There are several options to help you test DNS records, deliver ability, and encryption support.

### Dedicated tools

- swaks

### Dedicated websites

There are several handy web sites that can help you testing.

- https://mxtoolbox.com/
- https://ismyemailworking.com/
- https://www.mail-tester.com/ (3 free tries only!)
- https://www.checktls.com/
- https://pingability.com/zoneinfo.jsp

## Tips and tricks

Most mail servers can be configured to strip users' IP addresses and user agents from outgoing mail.

Available extras that can usually be integrated are:

- ClamAV for virus checking emails
- SpamAssassin to identify and filter spam
- Sieve – a mail filtering programming language
- webmail like Roundcube or Squirrelmail

Category:

- Mail server

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