Why won't this e-mail go through?

I received an e-mail address from an old friend and it won't go through. I tried sending them an e-mail using hotmail and gmail. Whenever I do a reverse e-mail look up it comes up as being valid and the IP address that they give me matches up with the location of where they currently live, so it definitely seems like it's a real e-mail address. However, every time I try to send them an e-mail, it gets sent right back to me and told me that it was unable to go through. Anybody have any idea why this is?