Add Outlook alias on iPhone mail app

I have had the same Outlook mailadress for years, but recently I changed my primary alias. I want this new alias to be the default mailadress from which I am sending my emails.

I have done this successfully on the mail application on my Macbook (via mail preferences), but I can’t do it for the iPhone app. I have tried several things. First, I tried to simply change my emailadress in the mail section in system preferences. I could do this and tap done, and the mailadress changes, but then after a second or 2, it changes back to the old alias.

I then removed the account from the iPhone app and re-added it, but the problem stays the same. When I send emails, it is still sent from my old alias and not the new one.

To make it all clear: I am talking about an @outlook.com adress. Can someone please tell me how I make sure that when I send mails from the iPhone app, it is sent from my new alias, like it is already the case for the macbook app?

Thanks in advance!

Eva