Mail server: Difference between revisions

Revision as of 15:58, 19 August 2018

A mail server or 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 or mail retrieval agents, you need to run a POP3 and/or IMAP server.

Software	Package	MTA	MDA	POP3	IMAP	SPF
Sendmail	sendmail^AUR	Yes	No	No	No	through Milter
Exim	exim	Yes	Yes	No	No	experimental[1]
OpenSMTPD	opensmtpd	Yes	Yes	No	No	No
Postfix	postfix	Yes	Yes	No	No	Yes
Courier	courier-mta^AUR	Yes	Yes	Yes	Yes	Yes
Cyrus IMAP	cyrus-imapd^AUR	Yes	Yes	Yes	Yes	?
Dovecot	dovecot	No	Yes	Yes	Yes
UW IMAP	imap	No	Yes	Yes	Yes
fdm	fdm	No	Yes	No	No
Procmail	procmail	No	Yes	No	No

MX record

If you want to receive mail, you need to set an MX record of your domain name to point to your mail server. Usually this is done from the configuration interface of your domain provider.

A mail exchanger record (MX record) is a type of resource record in the Domain Name System that specifies a mail server responsible for accepting email messages on behalf of a recipient's domain.

When an e-mail message is sent through the Internet, the sending mail transfer agent queries the Domain Name System for MX records of each recipient's domain name. This query returns a list of host names of mail exchange servers accepting incoming mail for that domain and their preferences. The sending agent then attempts to establish an SMTP connection to one of these servers, starting with the one with the smallest preference number, delivering the message to the first server with which a connection can be made.

Note: Some mail servers will not deliver mail to you if your MX record points to a CNAME. For best results, always point an MX record to an A record definition. For more information, see e.g. Wikipedia's List of DNS Record Types.

TLS

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

To obtain a certificate, see OpenSSL#Certificates.

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. To validate incoming mail using SPF you need to configure your mail server to use a SPF implementation. There are several SPF implementations available, perl-mail-spf and perl-mail-spf-query can be found in the official repositories.

For Sendmail spfmilter-acme^AUR can be used.
For Postfix, see Postfix#Sender Policy Framework.
Courier Mail Server natively supports SPF.

The following websites let you validate your SPF record:

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 dkimproxy.

Testing websites

There are several handy web sites that can help you test DNS records, deliverability, and encryption support.

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

@@ Line 4: / Line 4: @@
 {|class="wikitable sortable"
-! Software !! Package !! [[Wikipedia:Message transfer agent|MTA]] !! [[Wikipedia:Mail delivery agent|MDA]] !! [[Wikipedia:Post Office Protocol|POP3]] !! [[Wikipedia:IMAP|IMAP]] || [[#Sender Policy Framework|SPF]]
+! Software !! Package !! [[Wikipedia:Message transfer agent|MTA]] !! [[Wikipedia:Mail delivery agent|MDA]] !! [[Wikipedia:Post Office Protocol|POP3]] !! [[Wikipedia:IMAP|IMAP]] || [[SPF]]
 |-
 | [[Sendmail]] || {{AUR|sendmail}} || {{Yes}} || {{No}} || {{No}} || {{No}} || {{G|through [[Wikipedia:Milter|Milter]]}}
@@ Line 69: / Line 69: @@
 === Sender Rewriting Scheme ===
-The [[Wikipedia:Sender Rewriting Scheme|Sender Rewriting Scheme]] (SRS) is a secure scheme to allow forwardable bounces for server-side forwarded emails without breaking the [[#Sender Policy Framework|Sender Policy Framework]].
+The [[Wikipedia:Sender Rewriting Scheme|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]].