Email sent from wrong account

I have a big problem with my email.

I have 4 email accounts, each comes with IMAP and SMTP settings. When I send an email and select that e.g. the email is to be send from account 3, it will wrongly be sent from account 1. This is very embarrassing because I strictly want to seperate email accounts.

This behaviour is only on my Mac, on my iPhone it works fine.

What's wrong here?

Thanks!