Thanks for posting your question in the Microsoft Q&A forum.
After successful Exchange mixing, users cannot send emails on behalf of a list of communication groups.
Since the error message is missing, could you please provide the specific error message.
Here is my personal advice:
This issue occurs because the attribute () is not currently written back to the on-premises Active Directory Domain Services. This is by default.publicDelegatesGrantSendOnBehalfTo
You can refer to this official document for details:https://zcusa.951200.xyz/en-us/exchange/troubleshoot/send-emails/delegate-cannot-send-on-behalf-of-after-migration?source=recommendations
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