Use from address set in config for all emails

I totally disagree with this solution! A module should be able to modify the "from header".
The SMTP module setting for the "from header" should be used only as a fallback, otherwise it overrides the behavior of all modules!

The point is for it to override the behaviour of all other modules. Some SMTP providers require emails to be sent from a single "From" address. That is also what the field description says it does. If that behaviour isn't wanted, then the field needs to simply be left blank.