I am having THIS EXACT PROBLEM.
I have a professional email that I use to send and receive email for work and other purposes, and personal emails for everything else, which generally only receive email. I write, almost all of the time, from the one email. This was the way things had gone for years. Now, Mail is changing the account it is sending from, even after I disabled the other accounts, "broke" the SMTP information deliberately on the personal accounts and checked "use only this server" on every one of the accounts.
Mail still switches the sending account, without my choosing it, completely arbitrarily. Last night I needed to send a professional email and I wake up this morning to find it has sent from my personal address, WITH THE ACCOUNT INVISIBLE, DISABLED, NO LONGER AVAILABLE OR USEABLE TO ME FROM WITHIN MAIL, IT STILL SENT OUT THE MESSAGE FROM THAT ADDRESS. I am so disgusted. I had installed Opera and been using that browser for the personal email accounts. I am absolutely certain I was writing in Mail's window because of who I was writing to. I chose it deliberately. IT STILL SENT FROM THE PERSONAL ACCOUNTS. They were not visible, nor enabled.
The only solution I can see is what I've done now, delete those accounts entirely from Mail. When I looked at the list of SMTP servers, they were still in there, even with the accounts gone, so I took those out, too. I'll be deleting Mail entirely, even with the small problems with Opera, if it finds that old, deleted information somehow, somewhere else and continues to send from wrong addresses. It has done this several times now, in spite of every fix I have tried, upending everything to try those fixes.
I tried the link for the feedback, the webpage didn't show Mail, maybe no one wants to hear it.
At any rate, Opera has some nice features Mail doesn't have. It's got a few annoying issues, but so far, hasn't sent out mail from any email address it chooses.