Problem:
Emails may be going through your external spam filter (eg Barracuda) but not showing up in the inbox. Further review shows that they’re getting stuck in the quarantine.
Solution:
This solution is valid until Microsoft moves things around again.
First thing to do is to check to see if the email has been blocked/quarantined:
1. Go to EAC https://admin.exchange.microsoft.com/#/messagetrace
2. Assuming it’s been blocked, you can go ahead and create a daily notification
Daily Notification:
1. Go to Quarantine Policy https://security.microsoft.com/quarantinePolicies
2. Add custom policy and call it anything you want (I called mine UserQuarantinePolicy)
2a. Set Specific Access (Advanced)
2b. Block sender and Allow sender
2c. Enable with the first option selected
Setup Your Policy to Notify
1. Go to your anti-spam policy
2. From admin console, Security>Email & Collaboration> Policies & Rules > Anti-spam inbound policy (Default)
3. Scroll down to Actions (it should show spam message action etc.) and click on edit actions
4. Scroll to Select Quarantine policy and change the drop down to the one you created earlier (UserQuarantinePolicy)
Note that it could take up to 24 hours for it to go into effect.