For handling outgoing e-mail (stuff your server likes to send out), "apt-get install mailutils" will bootstrap everything quick and easy. (
See here for more on that)
For handling incoming e-mail (you're example.com and you want to receive mail for
bob@example.com), it's not as easy. Simply getting it to a state where mail arrives in a mailbox and can be picked up by a mail client is a pain, and that's without spam and virus filtering. It can be done, but outsourcing to Google or
Rackspace will save a lot of time and effort.
For firewalling, sure. If you're using Ubuntu (I've kinda been assuming that already), 'ufw' is dead simple. Really just:
Code:
# ufw allow openssh
# ufw allow apache
# ufw enable
# ufw status
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