I have a Fedora Core 3 system running Sendmail / POP3. My Linux mail setup experience is fairly limited - I know I'm using Sendmail, but Postfix is installed, too. I'm not sure whether the two interact, or if I'm only using Sendmail - it's however Core 3 is configured by default. (Sendmail shows up on a PS -A.)

A friend of mine has asked me to setup an account for his ten year old son, and limit the account to only be able to receive mail from, and send mail to, his parents. How do I configure the system to only allow mail from their two internal IDs, and deny any mail received from the outside? (Him sending to the outside is less of a concern - they just don't want him receiving the copious porn messages the rest of us are forced to battle.)

Any help you can offer would be greatly appreciated.

Might be overkill for this, but how 'bout spamassassin:

http://spamassassin.apache.org/