I was doing some troubleshooting with external recipient mail flow from an Exchange hybrid setup.
When I tried to send mail to an external domain I got "A matching connector cannot be found to route the external recipient" from the Queue Viewer on the Exchange Server.
This was caused by an incorrectly configured Send Connector on the Exchange Server, the Exchange Hybrid Wizard creates a new Send Connector which pushes all mail to your onmicrosoft.com. This will cause a problem if you are trying to send mail to external receipents from an on premise mailbox.
To resolve the issue, create a new Send Connector, using the SMTP space * (all domains) and scope it to your Exchange hybrid server, after this external mail flow will start working.