Hi Richard,
To sort out this issue, may I confirm some detailed information?
1. Is “Exchange: Database is mandatory on UserMailbox” just the whole error message? Is there any additional information? To know clearly about this error, could you please post a screenshot of the message?
2. About “These are some users that are synced with the on-promise site and having their email on office 365” you mentioned, I’d like to confirm the detailed environment to help analyze this issue? Here are my assumptions:
a. These users are synced from local AD to Office 365 via DirSync, and then you assign Exchange Online licenses to the users to provision the cloud mailboxes.
b. You have deployed Exchange hybrid, and then moved these mailboxes to Exchange Online.
c. You have done a staged migration.
Which one is the situation? If anything is misunderstood, please feel free to correct me and provide accurate information.
3. You’re right. Based on the message, this issue is related to the Database attribute. Do you mean you have verified they have the valid Database attribute via PowerShell? For example,
4. For these affected users, can they access the Exchange Online mailboxes as normal?
After confirming the information above, we will do some further analysis.
Thanks,
Young Yang