Hi,@Nicholas
Thanks for posting your question in the Microsoft Q&A forum.
Your understanding is correct that in a hybrid Exchange environment, Exchange Online and Exchange on-premises should be able to communicate with each other seamlessly. If you're able to send emails from Exchange Online (Email A) to Exchange on-premises (Email B), but not the other way around, it suggests there might be an issue with the configuration.
When you are in a Hybrid deployment, then you have to create the AD Account locally, ideally using EAC using the option to create an Office365 account. Then let the account sync to the cloud and licence it. If the entire account and mailbox was created in the cloud (so there is no local AD account) then it will not work correctly.
You can refer to this link for details:https://learn.microsoft.com/en-us/exchange/troubleshoot/email-delivery/ndr/recipientnotfound-ndr?source=recommendations
If my answer is helpful to you, please mark it as the answer so that other users can refer to it. Thank you for your support and understanding.