I'm not sure I follow the maintainer's reasoning...
#369666: "mailx is recommended, and that is the proper dependency, as it by default need mailx, but can be configured not to need it."
Umm, call me crazy, but if you ship the package needing it by default, I call that a dependency. In my humble opinion, that bug should be release critical, as it renders the package unusable in its default installed state.