Exchange online for relay

Brass Contributor
We have a separate o365 tenant and several external azure subscriptions that run LOB applications. We have a inbound connector configured in exchange online to accept messages only from these applications via specific IP address. The accepted messages than relay to our production user mailboxes that has E5 licenses. Mail routing worked until March 21 and now we get “ email sent to wrong office 465 region” when application try to send using Microsoft SMTP relay which their scripts send to MX record.

We do not have any licenses for this tenant. Do I need any licenses to use this tenant to relay emails to our production tenant users in office365?
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