When someone attempts to send an email to an exchange server from an IP address listed in the global deny list, they receive the error 550 5.7.0 Access Denied from our exchange server. This error is not very helpful to them in determining what the problem might be. Is there any way to change this error message in the registry or somewhere else so that I can refer them to a website, or something similar? I know you are able to change the message returned by EMF by adding a key to the registry, but couldn't find anything about changing this particular message anywhere in the knowledgebase.

Which version of Exchange?

Exchange Server 2003