Change FROM email address

Hi,

I started the trial version of office 365 small business today, I'm the only user.

So I created an microsoft account, then I did all steps to send email from my domain, all working. But then I wanted to change the mail address from first@mydomain.com to first.last@mydomain.com. I tried everything, even changed my login for office365. Everywhere now is displayed "first.last@mydomain.com", but Outlook keeps on sending emails as "first@mydomain.com".

It seems impossible to change that. Can somebody help?

PS: also in the desktop version of outlook I can not change the mailbox name etc.

Help! Br, Thomas