The nagios-core package has mail-client/mailx as a dependency. My guess is that this dependency is not so much on mailx, per se, as it is on having either the "mail" or the "Mail" program available. Although we do make mailx available to our users, it's antiquated at best and requires a raft of compatibility packages to make it work. Better would be nail, or perhaps GNU mailutils, both of which are better maintained (and are in portage already). Ideally, these packages should all provide virtual/mailx (I gather that "mailx" is the posix term, but please correct me if I'm wrong), and then nagios-core could just depend on virtual/mailx.
Nagios requires "mail" and that it works the same as mailx. Whether it is mailx or not shouldn't matter at all. Truth be told, nagios can work with ssmtp provided it is configured properly.
all done and fixed. in portage...
