This is a little trick for Postfix, it lets you log the
to of all the emails postfix sends (or which pass through it if you run it as
a relay). It comes in handy when you need to debug an email issue and need to
confirm your mailserver has sent the message.
First create the file
/etc/postfix/header_checks and insert this into it:
/^subject:/ WARN /^to:/ WARN /^from:/ WARN /^Subject:/ WARN /^To:/ WARN /^From:/ WARN