Log says:
Jam is deprecated and has been removed; targets that use Jam must be upgraded to native targets. For more information on doing this, consult the Xcode documentation.
This software is from 2007. The project's homepage no longer exists. It's not surprising it fails to compile on current systems.
Looks like someone else has at least done some bug fixes and released a new version here:
http://multixden.blogspot.com/2014/10/new-gnumail-release-122.html
We could try updating the port to that version.
