An Alias is an additional email address against your mailbox that you can receive emails to. Emails sent to an alias will arrive to the main mailbox's Inbox as normal.
An Alias can be promoted to become the main email address, known as the Primary SMTP or Default address, of the account.
If you need to send as the additional address, it should instead be configured as a Distribution Group, which you can grant Send As permissions over.
Add an Alias
Change the Default Address
If you wish to change the user's login to the default e-mail address, tick the box marked 'Synchronize Service User's login'. Click 'Finish'. This completes the change of the default e-mail address.