Hi @Alex Rough
What is the current user type of this user John.Miller@contoso.com?
You can use the following cmdlet in Exchange Online Powershell to check:
Get-Recipient -identity John.Miller@contoso.com | fl RecipientTypeDetails
If it is usermailbox, you can either create a mail flow rule (Exchange/server side method) or an Outlook inbox rule (Outlook/client side method) to redirect the message to email@example.com.
Mail flow rule:
Outlook inbox rule:
If it is not a usermailbox, the mail flow rule method should still work for you in this case.
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