Ran into an interesting issue with one of our Exchange Online customer. Thought it probably worth sharing with the solution I found.
The customer has an Exchange Hybrid setup. Recently some of Office 365 Exchange Online users complain they cannot email to a particular on premise mailbox: [email protected] The user bascialy does not show up in Exchange Online GAL. The on premise mailbox is working fine and other on premise staff can send emails to it without issue.