We are migrating an Exchange 2010 cu17 (CAS/HUB separated to MBX roles) environment to exchange 2016. We are still in the coexistence phase with some mailboxes being migrated to 2016 cu6.
The problem is that, a transport rule exist for some users, when they send emails outside the org to request approval from a moderator. The moderator receives twice the email for approval, and he needs to accept it twice for the same email sent.
After a few tests, we noticed that when the sender is still hosted on 2010 that the problem occurs, not for 2016 mailboxes, wherever the moderator mbx is hosted.
I tried to recreate the rule on 2016, migrated to moderator mailbox to 2016, test it on a lab with older CU for 2016 and 2010 (CAS/HUB/MBX role), same same.
Any idea what is happening please? There is a lot of mailboxes to migrate, it will take time.