set email as domain name

set email as domain name

I can not get my email set to info@mydomain.com and it's driving me nuts! 

I can change the "display as"  but the email address still shows as  numbers. I'm on the verge of deleting the whole thing and starting again, Please help! 

I'm sure there is a step I must be missing and searching the forum just leads me to migrating mail which doesn't applicable.