Well ... I've already broken my Postfix installation. All I had to do was repair permissions! I noticed a bunch of Postfix related stuff in the repair log and, sure enough, I couldn't use my SMTP anymore.
After much googling I found this page which contains the following fix:
sudo chown -R postfix /private/var/spool/postfix
sudo chown root /private/var/spool/postfix
sudo chown root /private/var/spool/postfix
sudo chown :postdrop /private/var/spool/postfix/public
sudo chown :postdrop /private/var/spool/postfix/maildrop
sudo chown :postdrop /usr/sbin/postqueue
sudo chown :postdrop /usr/sbin/postdrop
sudo postfix start
Now it all works, but presumaby repairing permissions will break it again! We'll see.
Continuing in the spirit of experimenting with an 'always-on' internet connection, I've been playing with setting up my own FTP server courtesy of PureFTPd-Manager. This app makes the set up ridiculously easy, and after some teething problems with my firewall, all is well (time-saving tip: if you want your clients to be able to use passive FTP you will either have to open all ports from 1024 - 65535 on your server's firewall, or you will need to manually specify which ports your server will use for passive FTP, and then only open those ports on the firewall).