I think it would maybe be arguable if someone actually hacked the OP’s account and the emails really did come from their outbox, but spoofed email is a different thing entirely.
It seems more equivalent as a legal precedent to someone sending a forged letter from a nonexistent employee on similar looking letterhead. Or maybe someone showing up at the door and collecting payment wearing a stolen or counterfeit uniform.
If you think of it in legal terms, in a lawsuit say, the client would have to acknowledge the existence of a contract and an obligation to pay the supplier, and then somehow make an argument that a spoofed email from a third party that the supplier had no awareness of, that never entered the posession or control of the supplier at all, somehow invalidates that contract, or proves that the client has satisfied their obligation.
That’s quite a stretch.
Arguing negligence on the part of the supplier still wouldn’t do anything to satisfy the payment obligation, at best it would seem to be a counter-claim, saying they they suffered a loss because of the suppliers negligence, but then that’s a separate tort and the burden of proof would be on them.