Office365 connector

Andrew1972 
If the need of this function only is for one specific account, and the need is only to receive emails then I would create a inbox rule on the mailbox with the domain "@domainB.net" and redirect / forward the email to the email address with the domain "@domainA.com".
That way, you can receive the emails to the mailbox you are managing.
https://support.microsoft.com/en-us/office/turn-on-automatic-forwarding-in-outlook-on-the-web-7f2670a1-7fff-4475-8a3c-5822d63b0c8e

But, please be aware that best practice is to disable redirect / auto forwarding outside the organization. The tenant that holds the mailbox with the domain "@domainB.net" might not allow it. How ever, it's possible to allow you to do this action through an separate outbound spam filter policy if the administrator allows it.