The cron script of rkhunter uses /bin/mail to deliver reports. mail-client/mailx has such a executable and should be added as dependency. Reproducible: Always
virtual/mail stands for any package that provides /bin/mail. mail-client/nail and net-mail/mailutils provide it as well.
Since you mention /etc/cron.daily - it ought to RDEPEND on sys-process/cronbase as well if it installs into that directory, or the directory might not exist.
Even better...
Created attachment 217156 [details] This ebuild should have correct rdeps
I have included your improvments into my new ebuild for version 1.3.6, see bug # 295433
It's fixed in both 1.3.6 and 1.3.4-r3.