About this issue, I would suggest you take the steps below to help us narrow it down:
- Create two new mailboxes, one hosted on Exchange 2010, the another hosted on Exchange 2016.
- Try to migrate them to Exchange online, check whether could migrate successfully.
In a coexist environment with Exchange 2010 and Exchange 2016, we need to use Exchange 2016 as the Internet facing server. Because request could proxy from Exchange 2016 to Exchange 2010, but not vice versa.
I also wanted to ask. If I decommission Exchange 2016, I won't affect the hybrid configuration in any way. After all, it is configured for Exchange 2010.
Yes, you could uninstall Exchange 2016, it doesn't affect the hybrid configuration. But, it is still suggested to rerun HCW after uninstalling Exchange 2016.
Personally, if need to choose one Exchange on-premises for management, I would choose Exchange 2016, because it is safer than Exchange 2010. EAC is more convenient to use than EMC.
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