Default setup in /etc/cron.daily/rkhunter is: EMAIL_CMD="|mail -s \"${EMAIL_SUBJECT}\" ${EMAIL_RECIPIENT}" and without virtual/mailx this result in lost rkhunter output without any signs of this error (no /bin/mail) in logs.
+ 11 Apr 2013; Patrick Lauer <patrick@gentoo.org> rkhunter-1.4.0.ebuild: + Dependency fix for #398123