If you open the User record in the Microsoft 365 Admin Portal, you’ll see that Microsoft Exchange Service is not seen in the list of Services.
Now, let’s procure a trial for Exchange Online or any of the E3 / E5 plans which you plan to purchase based on your requirements. They have Exchange Online services available which will enable your Exchange Online mailbox. In this example, I’ll start a trial because I don’t want to buy for Enterprise purposes for the sake of this example.
Now, once I started this trail. And if you see the comparison chart above, you could even opt for other licenses that offer Exchange Online – say, Office 365 E3 Now, I can see that this license is available for me to assign to the user.
And when I select the above license, I will see the Service available in the list as well.
Save your licensing changes / preferences and let 5-10 mins pass for the services to be applied to the User. And once you reload/refresh the Outlook web app, your mailbox will be available or retained (if it was expired on an existing license.)