Two things you need for a start - the WAN needs to be able to access port 25 (if destination port 25 OK from outside), and you need an MX record on your nameserver(s) pointing to your server (similar to the A record you must have if your http is working).
What do you mean you have an email address set up? postfix by default will accept mail for valid Linux users, so if you have a user mark, it should receive mail for firstname.lastname@example.org
Edit /etc/postfix/main.cf to set mydomain to cougar.com, myhostname to whatever_your_hostname_is.cougar.com (these may not need to be set, since postfix will try and pick them up), and make sure mydestination includes cougar.com (again, this will probably be OK).
Make sure you start postfix (service postfix start) and starts on boot (chkconfig poastfix on).
Send yourself an email (try from the server first using:
mail -s test mark
type message, followed by control D
Make sure it gets delivered (use "tail /var/log/maillog")
Assuming that works, you need a pop/imap server. I'd suggest dovecot, and a simple "yum install dovecot" will sort that. It should work out of the box. Start it and make sure it starts on boot (see above).
You should now be be able to connect a mail client from a desktop and check mail (make sure ports 110 and 143 are open for pop/imap).
Now try sending a message from the WAN to yourself.
See how you go and come back with further questions.