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.
Tom Tech Blog