This post documents how I did set up Postfix 2.6 to relay all of its e-mail to GMail.
I used different sources to assemble what is described next. Worth mentioning are Getting Postfix to work on Ubuntu with Gmail, Gmail on Home Linux Box using Postfix and Fetchmail, Postfix Gmail SMTP Relay and finally Postfix TLS [...]

A LDAP directory offers a single, logically centralized, hierarchical store to keep data, like information about users, groups, mailboxes, services, etc. There are a number of applications out there than can leverage this LDAP store. For example, PAM can authenticate by checking against an LDAP directory, Cyrus-IMAP can check whether a certain user has a [...]

The file /etc/postfix/master.cf of postfix-2.1.5-4.2.RHEL4.x86_64 contains a hard-coded path to Cyrus-IMAP LMTP deliver agent, /usr/lib/cyrus-imapd/deliver. However, this causes problems on x86_64 systems since Cyrus-IMAP LMTP deliver agent path is /usr/lib64/cyrus-imapd/deliver.
Fixing this problem is easy: just edit /etc/postfix/master.cf and replace /usr/lib/cyrus-imapd/deliver with /usr/lib64/cyrus-imapd/deliver.
I have filled in a bug report against Red Hat’s Enterprise Linux 4:
https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=192601

In order to evade mail-relaying, unathorized users try to spoof the e-mail sender address in mail messages. One trick they use consists in making Postfix think the sender belongs to a mail domain in $mydestination (that the message comes from a domain belonging to the list of domains that Postfix considers itself the final destination [...]