Relay office 365 email via smtp during migration

I am about to move users to Office 365 email from an old hosted smtp/pop system.

The domain is configured for 365 with the exception of the mx record.

I would like to create the accounts in 365 ahead of the migration and gradually move users over without impacting their current mailboxes.

For the first two test users I have forwarded the old mailbox to the new mailbox using the onmicrosoft.com alias. They receive these forwarded mails to their 365 mailbox. They can work.

A third test user is created in 365 but continues to use his non 365 mailbox until I'm ready to move him.

If the first test user emails the third test user, the email is routed internally within 365 rather than going out to the server in the mx record.

I created and validated a connector to relay all mail via the old smtp server in order for mail from the first user to reach the third test user. This does not work, the email goes to the 365 mailbox. As the third user is not ready to move, it can now not see email from the first test user, only email that did not originate from my 365.

My question is:

Can I use the connector to relay email for addresses that exist in my 365? Or as these are technically internal will they always go directly to the 365 mailbox?

It seems there may be no option but to change the mx and fully cutover to using the 365 mailboxes.

Thanks for any thoughts.

If it's a small business I would tend to cut over directly by changing the MX after prefeeding everyone mailbox PST content to O365 as you have less risk to loose a email in transit.

At the MX cut the Outlook client will stop getting new emails, but the users can use their O365 online via the Microsoft webpage to see their new emails for the time you connect their Outlook to O365.

You would just need to migrate their mailbox's content that wasnt synced (if the feed took a long time to do), but they loose nothing as the local PST will stay be available.

For the prefeed, see there for the step; Use network upload to import your organization's PST files to Microsoft 365 or, if big; Use drive shipping to import your organization's PST files