Licensing

It's all Free Software:

Sendmail
BSD License
ftp.sendmail.org/pub/sendmail/LICENSE
But, dual licensed by Sendmail, Inc (a bit like MySQL AB)
www.sendmail.org/license-info.html

Perl
Perl Artistic License
www.perl.com/language/misc/Artistic.html

MIMEDefang
GNU General Public License
www.roaringpenguin.com/penguin/open_source_mimedefang.php