Steps to reproduce:
1. emerge -C mailwrapper
2. emerge mailx
2.5. USE=mailwrapper emerge virtual/mta
3. echo Test | mail -s Subject firstname.lastname@example.org
mail: /usr/sbin/sendmail: No such file or directory
Can't send mail: sendmail process failed
mailx has no mailwrapper USE flag; that flag is for MTAs only. Either the MTA supports USE=mailwrapper and then it RDEPENDs on it, or it doesn't and then it will install /usr/sbin/sendmail.
Your steps to reproduce don't make sense.
My bad. It was a problem with an outdated qmail that I had prevented from upgrading. So there was nothing to pull the mailwrapper dependency.
OK, closing. Thanks for reporting back.