Mail will be sent from wrong account

Hello!

I can't get this to work. It looks like a bug but I can't quite believe it since it's too obvious.

I have setup multiple accounts in Apple Mail (IMAP). Each account has their own SMTP server to send emails.

When I compose a new email I select the account from where I want the mail to be sent from. However, it will deliver the email from the wrong account! I tried several times, the email will always be delivered with the wrong sender account.

It does work fine on the iPhone but it will not work on the Mac. I tried rebooting the machine, but the problem persists. What can I do?