PHP question (Flash contact form)

Does anyone happen to know why that would I only get emails when I use my own email address when testing my Flash contact form? I found out from my ISP that messages aren’t sent if the from email address isn’t from the same domain. I’ve heard that on some forums. Why would that be? That sounds stupid because the form should work for any visitor to the site.

Does anyone know a way around that?

Thanks everyone!