In Office 365 Exchange Online you may create an alias on a user in order to “translate the username better” e.g.
for the user firstname.lastname@example.org might not make sense and mabye the name somehow does not Sync correctly which could make it hard to find the user.
Add an Alias: havard.Siegel@Haukeberg.com
When you share files you may look up the alias in the search dialog Box like this:
You add an alias in the Exchange Online Control Panel.
Select the user and click Edit Exchange Properties
Select E-mail Properties
Enter the prefered alias like; email@example.com
Make sure SMTP is selected
Check the Make this the default reply address
Click Save and wait for the changes to propagate on the account