A mail forwarding loop happens when mail get's forwarded around, uh, in a loop... For example, say you send mail to user@host1.com which is set up to forward mail to user@host2.com which is set up to forward mail back to user@host1.com and so on and so forth. It just gets passed back and forth, wasting bandwidth, until a mail server somewhere gives up and does something with it (usually bounce it back to the sender). Sendmail tracks the number of hops the message takes and after it reaches a certain number (25?), it bounces it. Postfix adds a Delivered-to: header to the message which it then checks before forwarding it to see if it matches the forwarded address. For example, if it gets sent to user@host1.com, postfix will add that to the header, then when user@host2.com goes to forward back, postfix will notice that it's a loop and bounce it, cutting back considerably on the travel time of the message. Of course this causes some problems, such as preventing you from bouncing a message to yourself but you gotta do what you gotta do...