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Add Shared Mailbox in Sophos Mail

Hello everyone,
we use Microsoft Exchange 2019 onPrem and Sophos AD Sync to sync all AD accounts, shared mailboxes and groups to Sophos.
Unfortunately, when I try to add a shared mailbox as a managed mailbox in Sophos Mail, I get the following error:

Email address and account do not match. (AS38)

Is there any way to add shared mailboxes to Sophos Mail? I couldn't find any instructions on how to do this in the Sophos documentation.

Thank you very much for your help.