When attempting to modify a shared mailbox that is in 365 but anchored in on premise AD, I am receiving the following error when trying to add a regular user to mailbox delegation:
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Running the Add-MailboxPermission cmdlet directly works without issue. I investigated the mailbox permissions on prem and in 365 and do not see the Builtin Administrators group anywhere, I'm not sure where Adaxes is picking this up from.