It would be nice to have ability to append some headers to email notifications. I meant 4th parameter to mail() function ( http://www.php.net/manual/en/function.mail.php ).
In my opinion, it should be customizable either from AdminCP or from config.php
For example, when I've got non-latin symbols in task/user names or localized strings I have to add encoding information into email header. Something like "\r\nContent-Type: text/plain; charset=utf-8".
Now this task cannot be done without patching the source.