I don't really care for Exim - purely a subjective matter - and are a bit more comfortable with Postfix.
For this reason, I swap out the Exim mailer and install postfix instead.
Debian has made the exchange of MTA (Mail Transfer Agent) pretty straightforward, and as I have Yet to configure anything which involves a mail-server, there should be no trouble with just changing the default MTA.
To install Postfix we need to add some packages
apt-get install postfix apt-get install webmin-postfix
And that should be it...
Some configuration has been left behind by the Exim mailer, which is of course not a good thing for our nice little system.
To clean up we need to go a bit behind apt-get, and do some of the things a bit more manually.
dpkg -r exim dpkg --purge exim
If You would like to try out the maildir format, then fortunately postfix support this format directly.
In /etc/postfix/main.cf add a line with
home_mailbox = Maildir/
And we should be using Maildir when postfix is restarted/reloaded.
To allow mail to reach our mail-server, we need to allow connection to port 25 through our firewall.
Earlier a script was provided for creating firewall rules. Alter the script, so that it shows something like
EXTPORTSOK=" tcp:ssh tcp:ftp tcp:smtp "
What we have done is added the line with "tcp:smtp" to the script.
Rerun the script, and save this as the new active ruleset. If in doubt, refer back to Starting the firewall at boot.