Comment 2 for bug 1172187

Remove qmail packages by using Synaptic Package Manager (mark for complete removal) all packages: "qmail, qmail-tools, qmail-runs, qmail-uids-gids fast forwards", click <Apply> and close.

After a reboot you've noticed that you have different other sessions like Allias, qmaill, qmailp etc.
Login at your account and remove the other users (select delete files of accounts) with "User Accounts" or in terminal. After that, in terminal just check: sudo apt-get autoremove && sudo apt-get clean.

Reboot

On synaptic package manager (again) search for qmail on quick filter. now see "non installed (residual config)" and you'll see qmail package even that it isn't installed the cofiguratioon is there. right click the ceckbox and select (mark for complete removal) once again.