Noticed mails are getting queued in the Office 365 towards on-premises Connector and mails are get not getting delivered to any on-premises users when you have Hybrid Exchange setup.
you can see the queue using the https://protection.microsoft.com –>Mail Flow –> Dashboard
1. Check any network related issues if any problem in the Ports / Reachability to EDGE Servers configured as Hybrid Mailflow Servers.
2. Check all the certificates assigned for the TLS Communication and validate.
3. Check the CRL Validation is fine or it is showing the RevocationCheckFailure, If you see this error, refer the article to solve the issue.