SMTP - Should I?

OK, so I have this server running Samba, NTP, IMAP, and SSH. Now, I'm thinking about SMTP (Not backups, as I should be).

The current setup has each mail client pointing to the InsightBB SMTP server, and the mail goes out just fine. However, I'd like to learn more about email, so maybe I should setup one here. That way, we could begin the process of sending mail even if the connection is down (Wait! Isn't it an 'always on' connection?).

Also, when I send a message to my wife or kids (only a few feet away). It can take up to 20 minutes to appear in their inbox. We have this convoluted system of forwarding and fetchmail to get things to the IMAP server.

So, how does this thing work? I should probably first do a little research to see how to setup SMTP here. Then, do I tell my SMTP to contact the outside SMTP to get a message out? How about the internal messages? They will be addressed to an outside server, so I'll have to somehow fool it to keep those messages local.

Oh boy! A new project!