POP: mail.yourdomain.com
(substitute your domain in there)

SMTP:  mail.yourisp.com
(substitute what you normally use to send mail with your ISP).

About 50%-60% of all mail delivery systems, including AOL, hotmail, yahoo, Earthlink, etc., cross-check the sending and receiving mail servers of each mail message with an "anti-spam" system called ORBS (http://www.orbs.org).  ORBS is a database that is automatically maintained (read: there is no human to call). ORBS randomly checks mail servers around the world and employs a number of questionable techniques to gauge whether or not those mail servers are vulnerable.

If it suspects vulnerability, it immediately "black-lists" those mail servers.
The  problem is that ORBS is an automated system, with zero notification, and little documentation, guidance, or accountability. Mail servers can easily get wrongly black-listed, and it takes time for the ORBS system to self-correct.

So, if we opened up our mail relay for our customers to be able to use mail.theirdomain.com, then our mail server would eventually be shut out and no ISP would accept anyone's mail that was being sent from our server.