Sending and Receiving Email on an xxx.onmicrosoft.com Address — How Licenses and Mailboxes Fit Together

Create a Microsoft 365 tenant and you get an address like `user@contoso.onmicrosoft.com` automatically, yet when you try to send or receive on it, mail may not even arrive. The reason is that the Entra ID account you sign in with and the Exchange Online mailbox that stores mail are two separate things. This post walks through why it doesn't work by default, which license to buy and assign, and the outbound sending cap that starts biting onmicrosoft.com senders in 2026, following the actual admin steps.