Hi @ExUser44 ,
Does your application work according to using Outlook profile? If so, your solution is feasible, when assigning the "Full access" permission of the on-premises mailbox to the mailbox in Exchange Online. The on-premises Exchange mailbox will be added to the Outlook profile of Exchange online mailbox. Then the mailbox auto-mapping feature uses the Autodiscover to automatically open the mailbox in the delegate’s Outlook profile. But “Full Access” allow the delegate to open the mailbox, and view, add and remove the contents fo the mialbox, so please pay attention to the information security of your on-premises mailbox after assigning permissions.
In addition, considering that you are in an Exchange Hybrid environment, migrating mailboxes to Exchange online is also a solution.
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