[dfrussell]

Reverse lookups are not a good idea because many people use return email addresses not associated with the sending IP. A good example is all the people with yahoo/hotmail IDs and sending email out from their place of employment.

[KoRn]

That's exactly right. Someone using an email client from work pulling messages from a different server appear to be spammers. That has been the source of many of a headache for me. If I had my way people at the company would only use their email clients for company email, and would only use their company email for company purposes. Too bad that decision isn't up to me :-(

Most providers have web based email that can be used by their customers.