How does the object grabbing the emails from the pop3 inbox know what user sent them?  Is it by the email address or the generated key in the subject line or both?

If somebody is using Outlook and hits respond the "RE: serial key" isn't put in the subject line I don't think.

I would think it would be done by email address.  If so, what if a user has multiple usernames with the same email addy?  Which one will be picked?

Does each forum require a different POP3 email address?