We got these error messages on a few of our postfix servers:

warning: database /etc/postfix/virtual.db is older than source file /etc/postfix/virtual

warning: database /etc/postfix/transport.db is older than source file /etc/postfix/transport

These messages appear whenever one of these databases/tables is changed.

The optional transport table specifies a mapping from email addresses to message delivery transports and next-hop destinations. Message delivery transports such as local or smtp are defined in the master.cf file, and next-hop destinations are typically hosts or domain names.

The optional virtual alias table rewrites recipient addresses for all local, all virtual, and all remote mail destinations. This is unlike the aliases table which is used only for local delivery.
The main applications of virtual aliasing are:

• To redirect mail for one address to one or more addresses.
• To implement virtual alias domains where all addresses are aliased to addresses in other domains.

Issue these commands depending on the messages you get:

postmap /etc/postfix/transport
postmap /etc/postfix/virtual

Note: Although everyone suggests you should restart postfix after the command I didn't and the messages went away. If you need however to restart postfix you could do that like this:

service postfix restart

http://www.postfix.org/virtual.5.html
http://www.postfix.org/transport.5.html