nasty postfix mapping problem
 

I've encountered a rather pesky problem in my postfix configuration. I've set up several users and some canonical mapping entries to rewrite the username@domain.tld to firstname.lastname@domain.tld.
Everything works beautifully except one problem: the Reply-To header set in my client program (be it evolution, outlook or sending mail from python or php) is being rewritten and gets replaced by username@domain.tld

I find this rather annoying, since it renders the mapping useless.