Mail flow from on-premises devices and applications to Exchange Online is a tricky topic. The documentation allows for different solutions.
Recently a client ran into a situation where an on-premises application was not able to deliver messages to a configured inbound connector in the Exchange Online tenant. The connector was configured for remote IP address selection.
Exchange Online responded to each connection attempt with the following error message:
There weren't any changes on the on-premises configuration and the setup was in use for multiple months without any issues.
It took some time to identify the solution, but in the end, the solution was easy.
Disabling and re-enabling solved the issue.
Enjoy Exchange Online.