From http://www.postfix.org/postconf.5.html#canonical_maps,
Note: with Postfix version 2.2, message header address mapping
happens only when message header address rewriting is enabled:
* The message is received with the Postfix sendmail(1) command,
* The message is received from a network client that matches
$local_header_rewrite_clients,
* The message is received from the network, and the
remote_header_rewrite_domain parameter specifies a non-empty value.
It is not immediately clear whether these conditions are and/or. A
possible clarification would be to add "Header address rewriting is
enabled when any of the following are true:" prior to the list.
