I am curious to know if there are any major gotchas with setting up a 'reverse hybrid' 365 deployment, where all mail flow is already in the cloud with on-prem ad sync already enabled and simply adding on-prem exchange to the mix. I have found surprisingly little discussion about this online and I was hoping to find out if there are any major concerns with doing this sort of thing.
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2What's your purpose for this? Knowing that will give us some insight as to how to answer this question. – joeqwerty Aug 27 '21 at 21:55
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Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. – Community Sep 05 '21 at 02:37
1 Answers
Based on my research and test, if you haven't deployed on-prem Exchange, there will not be any Exchange attributes(e.g. msExchxxxxx) in local AD.
Besides, after you assign licenses to these synchronized users and install on-prem Exchange, on-prem AD will not fill these Exchange attributes and you will not see this user in on-prem ECP.
And this could also cause the inventory of your on-premises AD Connect domain and Azure AD domain to show incorrect data and conflicting information: Why Use These Methods to Create an Office 365 Mailbox?
To avoid any accidential issue, you'd better complete this deployment by the help of microsoft.
If you want to add a new on-prem Exchange for hybrid and manage these office 365 mailboxes via on-prem Exchange server, you need enable remote mailboxes for them.

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