In order to fight spam, our email servers have very high standards for accepting email. Occasionally, this may cause someone who wants to send you email from a misconfigured email 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.
Since most of the inquiries about this come from people who are not members but have email problems and don't know what to do about them, most of our documentation of this subject can be found on our public Email Acceptance policy page in an attempt to help them.