Added

• SendWithReversePath method to builder, allows specifying a reverse-path that differs from the from address (#179, thanks cgroschupp)
• A Sender interface that allows our users to provide their own mail sending routines, or mock them in tests. #182

Fixed

• Reject empty addresses during builder validation (#187, thanks jawr)
• Allow unset subject line during builder validation (#191, thanks psanford)

Changed

• Updated dependencies