Setting a default sender E-mail adress

Hi,

I's used to Outlook 2010, and there I can set a different sender/reply-to adress than the account name in my Hosted Exchange account.  In Office365 the the standard :  xxx@name.onmicrosoft.com  is set, and I would like to change that to : xxx@name.no 

How can this be done ?

Regards

Rune Mesel