virtual-package-depends-without-real-package-depends can be resolved by adding...

virtual-package-depends-without-real-package-depends can be resolved by adding a depends on bsd-mailx which is the real package that mailx transition depends on

debian/changelog

 backupninja (0.9.7-1) unstable; urgency=low
 
+  * Add dependency possibility on bsd-mailx for mailx transition
   * Bump standards version to 3.8.3 (no changes)
   * Install rsync and maildir examples (Closes: #566703)
   * Fixed upstream homepage and VCS in debian/control (Closes: #525318)

debian/control

@@ -10,7 +10,7 @@ Vcs-Git: git://labs.riseup.net/backupninja.git
 
 Package: backupninja
 Architecture: all
-Depends: gawk | mawk, dialog, bash (>=2.05b-26), mailx | mailutils
+Depends: gawk | mawk, dialog, bash (>=2.05b-26), bsd-mailx | mailx | mailutils
 Suggests: rdiff-backup, hwinfo, genisoimage, wodim, cdrdao, dvd+rw-tools, debconf-utils, mdadm
 Description: lightweight, extensible meta-backup system
  Backupninja lets you drop simple config files in /etc/backup.d to