Hi @David Turner ,
I still cannot receive email from internal or external sources.
Is there any bounce back message (NDR) to the sender when this occurs? If yes, you can remove all personal information and then share the bounce back error here so that we can help investigate further.
Besides, below is a KB article about a similar issue for reference:
Delivery has failed and 550 5.1.1 RESOLVER.ADR.RecipNotFound NDR after a mailbox is moved to Microsoft 365
You can go through it and try the solutions provided there to add the target email address address supposing the symptom applies to your situation.
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