Forum Discussion
Adding a new domain (company acquisition) to existing Office 365 Environment
When you add the domain, you need to make sure to configure it as Internal Relay in Exchange Online. Otherwise, for any address associated with that domain, Exchange Online will attempt internal delivery, and will fail if no matching account was found. When you set the domain as Internal relay, it will still check for internal matches, but if no such are found it will redirect the message to where the MX record points at currently.
adam deltinger license is not a requirement generally speaking, as you can have valid Exchange recipients even without a license. But if a matching recipient exists, ExO will always try internal delivery first.
Connector might not be needed as well, depending on the configuration. And after all 200 users isn't that much, you can probably do the migration over a single weekend, and avoid dealing with complicated setups.
