Postfix feature overview

Supported environments

Postfix runs on AIX, BSD, HP-UX, IRIX, LINUX, MacOS X, Solaris, Tru64 UNIX, and other UNIX systems. It requires ANSI C, a POSIX.1 library, and BSD sockets. In addition, Postfix requires that the file system satisfies a number of requirements that are described at the end of this page.

Main features

The following is a list of major Postfix features. Some features require third-party libraries (examples: LDAP, SQL, TLS). Other features are available only when the necessary operating system support exists and Postfix knows how to use it (examples: IP version 6, connection caching).

Junk mail control

Postfix 2.8	Postscreen zombie blocker.
Postfix 2.8	DNS whitelist support in smtpd and in postscreen.
Postfix 2.5	Stress-dependent configuration
Postfix 2.3	Sendmail Milter (mail filter) protocol
Postfix 2.2	SMTP server per-client rate and concurrency limits
Postfix 2.1	Access control per client/sender/recipient/etc.
Postfix 2.1	Address probing callout
Postfix 2.1	Greylisting plug-in
Postfix 2.1	SPF plug-in
Postfix 1.1	Content filter (built-in, external before queue, external after queue)

Protocol support

Postfix 2.10	HAproxy support in postscreen and smtpd
Postfix 2.9	Nginx proxy support in smtpd including SASL credential passing
Postfix 2.3	DKIM, DomainKeys and SenderID authentication (via Milter plug-in)
Postfix 2.3	DSN status notifications
Postfix 2.3	Enhanced status codes
Postfix 2.3	Plug-in support for multiple SASL implementations (Cyrus, Dovecot)
Postfix 2.2	Connection cache for SMTP
Postfix 2.2	IP version 6
Postfix 2.2	TLS encryption and authentication
Postfix 2.0	MIME (including 8BITMIME to 7BIT conversion)
Postfix 1.0	ETRN on-demand relay
Postfix 1.0	LMTP client
Postfix 1.0	Pipelining (SMTP client and server)
Postfix 1.0	SASL authentication
Postfix 1.1	QMQP server

By popular demand...

Postfix 2.9	Non-repeating (long) queue IDs
Postfix 2.8	Server reject "footer" text
Postfix 2.3	Configurable delivery status notification message text
Postfix 2.3	Sender-dependent SMTP relay lookup
Postfix 2.3	Sender-dependent SASL password lookup

Database support

Postfix 2.11	LMDB database
Postfix 2.10	Sendmail-style socketmap
Postfix 2.9	Gradual degradation: many cases a Postfix daemon will log a warning and continue providing the services that are still available, instead of immediately terminating with a fatal error.
Postfix 2.9	Memcache database
Postfix 2.8	SQLite database
Postfix 2.2	CDB database
Postfix 2.0	PostgreSQL database
Postfix 1.0	LDAP database
Postfix 1.0	MySQL database
Postfix 1.0	Berkeley DB database
Postfix 1.0	DBM database

Mailbox support

Postfix 1.0	Maildir and mailbox format
Postfix 1.0	Virtual domains

Address manipulation

Postfix 2.2	Masquerading addresses in outbound SMTP mail
Postfix 2.2	Selective address rewriting
Postfix 1.1	VERP envelope return addresses

Postfix file system requirements

The Postfix mail queue requires that:

Renaming a file to a near-by directory does not change the file's inode number.
A file is not lost after fsync() of that file (not its parent directory) returns successfully, and then the system crashes. This must be true even when that file is later renamed to a near-by directory.
When Postfix in a virtual guest machine flushes a file with fsync(), the file information must not be cached in volatile host memory. Instead the information must immediately be written to disk (or to persistent cache) before fsync() returns in the virtual guest machine.
Postfix can set the execute bit on a queue file. If this does not work, then no mail will ever be delivered.

In addition to the above, Postfix maildir delivery requires that:

A file can be hard linked between different near-by directories.
A file is not lost when it is hard-linked to a near-by directory, unlinked from the old directory, and then the system crashes.

Postfix mailbox delivery introduces no additional requirements.

Files in the Postfix command_directory require that:

The setgid bit works. This is required to access the mail queue with the postdrop command, and to access protected UNIX-domain sockets with the postdrop and postqueue commands.