This is more a feature request, than a bug.

It would be nice to be able to set up an outgoing mail filter to be able to
set custom headers, or perhaps right in the composition window.  The main
thing I'm looking for this for is Mail-Followup-To:

being able to set a default MFT when sending to a mailing list (in my case,
qmail@list.cr.yp.to) would be incredibly handy, and a nice feature to add
to evolution.

Also, the ability to override this default with a Mail-Followup-To option
box in the composition window would be great, or perhaps that would be an
option in the rule (allow composer to override default)

Thanks, and keep up the good work, Evolution rocks :D