Many businesses use alias addresses for their users, allowing multiple email addresses to be associated with a single mailbox. For example, email@example.com also needs to receive emails sent to firstname.lastname@example.org.
To configure user aliases, login to the Office 365 Admin centre, as detailed here: https://neural.zendesk.com/hc/en-us/articles/217622427-Logging-in-to-Office-365-Admin-Portal
Once logged in, go to Users > Active Users and select the user you wish to configure an alias for, then on the right, choose Edit
On the edit menu, choose Email address, then select Add New
Enter the email prefix you wish to associate with this mailbox, then choose the domain from the dropdown. You can elect to use this address as the user's primary address, which will mean they now use this address to login, and that this is presented by default on all outbound emails. Click Add to complete.
If you have a secondary production domain you wish to use, but isn't listed here, please follow these steps to add an additional domain: https://neural.zendesk.com/hc/en-us/articles/217168588-Adding-a-Secondary-Production-Domain