Sometimes viewing the Queue viewer inside System Manager, i notice the are tons of domains that i or anyone in the org do not send to. I do not know who summitted them. When i open a queue and click on find messages, the sender is usually not a valid email address in exchange. My question is how does an email get into exchange and how can i find out who computer is it coming from?

This is most likely due to SPAM. SPAM was sentan invalid address within your organization. Your server is trying to send back an NDR.