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Unable to Email Some Internal Users

We recently changed our Exchange environment to run in Hybrid mode. The goal is to move all users to Exchange online then decommission our on-prem environment once everyone has been migrated. We use the latest version of DirSync (not federated services) so our users can authenticate to Office 365 services using their internal AD credentials. 
As my team began migrating some pilot users to Exchange online mailboxes, we found that once a user had be migrated to an Exchange online mailbox they were unable to email some of our internal users (about 25 out of the 3000 users). 
Our Microsoft partner had no clue how to resolve the issue and we opened several cases with Microsoft wasting many hours of time with no good results. Our Microsoft partner eventually said we would need to remove those 25 users from DirSync and then re-add them. This meant 25 users would need to take all their documents out of OneDrive and then put them back in, plus they'd have to resetup their Lync groups and re-follow every document in SharePoint Online they had previously followed. Essentially it meant a huge pain in the butt for those 25 users which included 2 executives, as well as several senior managers and remote sales staff. 

Eventually we figured out a MUCH better fix:

1. Go to Office365 admin portal and assign an Exchange Online license to any user who cannot be emailed by another user that already is using Exchange Online. Generally this would not be a good idea because now the user has both an on-prem mailbox and an online mailbox and they would need to check both mailboxes in order to receive all their email. 
2. Wait 30 minutes for replication within Exchange Online
3. Ensure a DirSync synchronization has run in the last 30 minutes. if it has, run it manually. 
4. Remove the Exchange Online license for the user. This should delete the Exchange Online Mailbox so the user has only their on-prem mailbox again.
5. Wait 30 minutes for replication within Exchange Online
6. Ensure a DirSync synchronization has run in the last 30 minutes. if it has, run it manually.

Users which already have Exchange mailboxes should now be able to email the user without issue.


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