To fight spam, our email servers have very high standards for accepting email. Occasionally, this may cause someone who wants to email you from a misconfigured server to run into problems. If you can get a copy of the bounce message and forward it to us, we may be able to point the server operators in the right direction as far as fixing their problem. But since it is their problem, it will be up to them to fix it.
Most inquiries about this come from people who are not members of our service but have email problems and don't know what to do about them. In an attempt to help them, most of our documentation of this subject can be found on our public Stop Helping Spammers! page.