apt-get install ssmtp

[Alex Muntada]

+ Kaushal Shriyan <kaushalshriyan at gmail.com>:

> * Stopping MTA
>    /sbin/start-stop-daemon: warning: failed to kill 3668: No such process
> invoke-rc.d: initscript exim4, action "stop" failed.
> dpkg: error processing exim4-base (--remove):
> subprocess post-removal script returned error exit status 3

When I face this kind of problem I usually add and "exit 0" in top of
the script giving the error (i.e. /etc/init.d/exim4). The daemon is not
running and this script is going to be removed anyway.

HTH

-- 
Alex Muntada <alexm at alexm.org>
http://alexm.org/