Or, Diagnostic information for administrators: #550 5.1.1 RESOLVER.ADR.ExRecipNotFound; not found ##
NDR Error: Delivery has failed to these recipients or distribution lists
The recipient's e-mail address was not found in the recipient's e-mail system. Microsoft Exchange will not try to redeliver this message for you. Please check the e-mail address and try resending this message, or provide the following diagnostic text to your system administrator.
Possible Reason 1: - The user does not exist
Resolution: - Please check if the user email ID your typing is correct or not. Generally, this happens, if the user mailbox has been deleted or you are typing a wrong email ID
Possible Reason 2: - The user which have been deleted from your AD/Exchange, may still exist/added in some Transport Rules.
Resolution: - Please check all your Transport rules and delete the user email address from any transport rules that you have created under EMC > Organization Configuration > Hub Transport> Transport Rule
Cheers, please write me back if you have any query or feedback on this.