Edit: I’ve posted a video of this here
In a hybrid environment, when a mailbox is migrated from on-premise to Exchange online, it will retain its primary SMTP email address. But what about when you create a new user who has never had a mailbox on-premise?
When you assign an Office 365 license, and a mailbox is created, you might find that the primary email address is set to the Microsoft Online Email Routing Address (e.g. domain.onmicrosoft.com).