Un Authorize Domain User?

[JUNG2]

How can someone else use our domain without being added to our user list. Example: we would name our domain as @ourdomaine.com. We have all the user's name that can use this domain for email. For some reason we would get emails from someone that would say johndoe@ourdomain.com. We never had this email address added to our domain but he has exact domain name as ours. This is very puzzling to me. pls. help..
Thank you...

[LocoCoyote]

Could be a bunch of things. The first thing I would check is the email headers to see if the mail is indeed coming from your domain. Forging an email is a trivial task. For anything else, I would need to know what OS and version you are running and all that other stuff.