Do you have 2 persona's & 1 external account listed? Or 1 primary persona & 1 external account?
If there's 3, instead of making a new persona, set it in the external account itself.
See pics of the different options in external account vs persona.
In my case
user@domain.com is the account, & I've manually written in
test@domain.com (the external account) on the from fields.