Sometimes when you are working with Postfix you may be on an older version that does not support a feature you need.  Here is a list of the version and the major features that were added for that version.

Postfix 2.5 Stress-dependent configuration
Postfix 2.3 DKIM, DomainKeys and SenderID authentication, DSN status notifications, Enhanced status codes, Plug-in support for multiple SASL implementations (Cyrus, Dovecot), Configurable delivery status notification message text, Sender-dependent SMTP relay lookup, Sender-dependent SASL password lookup, Sendmail Milter (mail filter) protocol
Postfix 2.2 Connection cache for SMTP, IP version 6, TLS encryption and authentication, SMTP server per-client rate and concurrency limits, CDB database, Masquerading addresses in outbound SMTP mail, Selective address rewriting
Postfix 2.1 Access control per client/sender/recipient/etc., Address probing callout, Greylisting plug-in, SPF plug-in
Postfix 2.0 MIME (including 8BITMIME to 7BIT conversion), PostgreSQL database
Postfix 1.1 QMQP server, Content filter, VERP envelope return addresses
Postfix 1.0 ETRN on-demand relay, LMTP client, Pipelining (SMTP client and server), SASL authentication, Berkeley DB database, DBM database, LDAP database, MySQL database, Maildir and mailbox format, Virtual domains