The last couple of days I did some test with Exchange Online and Exchange on premise hybrid environments. I connected my on premise Exchange environment with Exchange Online and moved a mailbox to the cloud (Onboarding). The move succeeded and the mailbox was reachable in the cloud. After this I wanted to move the mailbox back to my on premise environment (Offboarding) and created a new move request. This time the move failed and I got the following error in the move log:
Error = Transient error ResourceUnhealthyException has occurred
The reason for this error is that the search index of my on premise databases was corrupt. Before Exchange starts the move of the mailbox it check if the destination mailbox is healthy. If this is not the case, because the search index is corrupt, the move will fail with error message above. I followed the instructions in the following articles to recreate the search index of my content databases:
The whole process of index recreation took some hours to complete. Afterwards I created a new move request and this time it worked and the mailbox was moved back to the Exchange on premise organization.