When sending message to the valid recipients using Outlook you may receive the following bounce back: Delivery has failed to these recipients or distribution lists:
'Contact_Display_Name'
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.
Diagnostic information for administrators:
...
#550 5.1.1 RESOLVER.ADR.ExRecipNotFound; not found ##
It can be caused by the following:
How to reset the Outlook nickname autocompletion cache