We are having issues with multiple authentication prompts when trying to open emails in HTML format. It does not happen on all emails just ones with embedded pictures and even then it is only a few.
We have run dcdiag agains the Domain Controllers and checked all the policies. We have also reset the computer account and the passwrd for the Exchange Server. Rebooted the Domain Controllers and checked the FSMO roles.
Also a rebuild of the RUS was successful.
DNS is clean and there are no issues there.
On new accounts we have also noticed that NDR(s) are being generated but, not all the time.
You do not have permission to send to this recipient. For assistance, contact your system administrator.
All eventlogs seem to be clean and advanced logging on the Exchange server show no relivant information.