Help! Issues with Exchange Hybrid Migration of Distribution Lists and Mail Contacts

Last night, we ran into problems during our Exchange Hybrid migration involving distribution lists and mail contacts. Currently, we have our mailboxes in Exchange Online but still maintain one on-premises mailbox along with 1,000 distribution lists and mail contacts.

I wrote a script to handle most of the migration tasks, which included creating equivalent objects in M365 with a name prefix, and then removing that prefix after we deleted the on-premises objects from the Azure Synced OU. Additionally, I renamed the on-premises objects to include this prefix.

We thought that after renaming everything in M365, all we needed to do was change the domain setting from Authoritative to Internal Relay. The plan was for any emails sent from the on-premises mailbox to fail locally but reroute to M365. However, to our surprise at 1 AM, this wasn't working as expected. Upon checking the logs, we saw no attempts to utilize the hybrid Send connector.

While testing, I discovered that we could create an on-premises mail contact with the original primary SMTP address and the mail.onmicrosoft.com external address for proper routing. Yet, I didn't believe this was the only method to ensure mail flow with just one on-premises mailbox in use. Am I missing something here? It seems we have either 1,000 mail pointer contacts to deal with or we need to migrate the last mailbox, but that's currently not an option due to the high volume of mail it's sending to Exchange Online.