Here are suggested steps to troubleshoot issues with emails not being delivered to a specific user in Exchange:

1. Check if the mailbox is available.
2. Check if there is enough disk space.
3. Check mailbox quotas.
4. Check mailbox rules.
5. Check if the mail flow to/from the mailbox is blocked due to any security rule.
6. Check mailbox forwarding rules.
7. Check the Exchange mailbox delivery restrictions.