We are running Exchange Server 2003 Enterprise. When all internal emails are sent successfuly. When sending to some external adresses, but not all, we get the following error:
There was a SMTP communication problem with the recipient's email server. Please contact your system administrator.
<xptjhbex.xpert.local #5.5.0 smtp;550 Rule imposed mail relay denied>
xptjhbex.xpert.local is our local server. This message comes through after we send, almost instantly.
We have checked all the SMTP relay setting and all is correct.
Does anyone know something else that could be causing this problem?