When unsubscribing the email of the receiver of the mass mail has to be taken, not any other. Currently the main address is used if the receiver is an alias.
For me, this is why I tend to right-click on unsubscribe links & open in an incognito window.
Many times, I don’t have (or can’t be bothered to set up) a way to send from the old email.
If eM Client were going to be able to send an unsubscribe email from the correct account, it’d have to be set up properly, etc.