In support, we hear from people in this “duplicate mailbox” situation relatively often. When this happens, our customers ask us for the best way to recover. In my experience, there are typically two ...
Published Sep 11, 2019
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Shaun Jennings I understand what you're saying, except I don't know how we can get them to match. The one scenario I can think of, but haven't ever seen so can't really confirm is the 2 mailbox scenario where the 2nd or most recently-created mailbox is an erroneously created on-premises mailbox for an already-migrated mailbox, or a cloud mailbox where the on-premises Remote<User/Shared/Room/Equipment>Mailbox had its ExchangeGuid set to match the cloud mailbox (i.e. the scenario described in this blog post in the "Duplicate (new) on-premises mailbox" section.
I really do hope that the Exchange Team can come back and see our conversation here, and hopefully help us with the other scenario (which I argued in my earlier comment is WAY more common), where the 2nd/new mailbox is created in EXO due to PPL (premature pre-licensing).
With COVID-19's impact on companies adopting Teams, this dual mailbox problem scenario is getting to be very common. The specific parameters of this scenario are: Customers jump into Office 365 and license all their freshly synced users. In that process, the purposely do not setup Hybrid Exchange, or enable the "Exchange Hybrid Deployment" optional feature in AAD Connect because aren't ready for EXO migration yet.
Even before COVID-19 though, I've seen very commonly that customers license new users too early, before the users are setup in on-premises with a mailbox or as a remote mailbox.
I feel like it may actually be worthwhile to open a user voice for a property on OrganizationConfig which can be set to turn off automatic EXO mailbox provisioning. That or a default behavior of automatically disabling the EXO P1/P2 app from within O365/M365 licenses. If I do, I'll come back here with the link to beg people for votes.