Is it possible to setup an SMTP and POP server on linode 80 with just a single Ip address? (I saw another thread where someone stated they had Qmail with linode 80, but I want to be sure before I start.)

Short answer: yes.

Slightly less short answer: yes because they use two different ports. POP3 uses port 110 and SMTP uses port 25.

I would highly recommend being very comfortable with how the following work before attempting to setup your own mail server:
TCP Ports
SMTP
POP3

and last but not least:

Open relays

Well put.

It's also a good idea to learn about SMTP AUTH and SSL.

If you can use an authenticated connection over an encrypted link, you don't have to worry about your ISP blocking port 25 (SMTP-over-SSL uses 465). Using POP3 or IMAP with SSL is also a win.

If you're only worried about providing service to yourself or a small group, you can make your own self-signed certificates (or, create your own Certificate Authority) and avoid forking over money to Verisign.

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