[desktop] Installation of mail server by default

[Matthew Forrester <fozz@gmx.net>]

I always thought the installation of EXIM by default was rather strange,
If you want a mail server your almost certainly going to want a specific
mail server, and after all your only going to want one per network. A
sensible Idea would be to have nullmailer installed. none of my debian
using friends knew about nullmailer, but were well chuffed when i told
them about it, as they could get rid of the mta on there workstations
(which serves no other purpose except a potential security problem, use
of hdd space and stopping apt complaining about not having an mta).
-- 
Matthew Forrester <fozz@gmx.net>