Email address Policies in MS Exchange 2016 ECP

I have two domains in my MS Exchange configuration in the ECP section under "Mail Flow" and called "Email Address Policies"

I currently see...

domain1.com Priority 1 Applied
then
domain2.com Priority Lowest Applied

I need to swap the two of them so domain2.com is the highest priority. What's happening is that when my users reply to an email, it's defaulting to send as domain1.com when I really want them appearing as domain1.com to recipients.

LASTLY - I only want SPECIFIC users to have domain1.com. Any easy way to do that? I'm sort of in a rush as I only have today to finalize this. I'm migrating all of our users.

Thanks for any help!!!

Ryan