Hey, Fred.
I see that emails you're sending from the Mail application on your Mac aren't being sent from the one account you prefer, and I'm glad to help.
Address emails in Mail on Mac -- Make sure that the "Send new messages from" setting is still pointing to the correct address.
Use safe mode to isolate issues with your Mac -- If the correct address was selected, restart your Mac in safe mode and test how it responds. If Mail works as expected in safe mode, reboot normally and test again in your own user account. If not, continue to "How to test an issue in another user account on your Mac" below. Remember this when booting in safe mode: Graphics performance limited in macOS Recovery, safe mode, or Guest User mode
How to test an issue in another user account on your Mac -- If the issue is also happening in safe mode, use this article to create and test a new administrative user account. This will demonstrate if the issue is isolated to your account, or if it's system wide. You’ll need to set up your email accounts in this new user profile to test.
Let me know how that goes, and take care!