I'm trying to get my emails to be sent in the format firstname.lastname@domain.com but have been unable to do so.
I created all the useraccount with firstname@domain.com and added the firstname.lastname@domain.com as an alias and then changed it to be the primary address.

It still shows firstname@domain.com as the sender for all the new emails I'm creating. 

I can of course choose firstname.lastname@domain.com in the dropdown list but I would like it to  be the default.

How do I go about that without recreating the accounts?