I would hate this, and probably instantly bail out of any flow that tried to force it on me.

Being able to receive mail on an address should not necessarily imply a capability to send from it. This scheme rules out many actual or potential schemes for email anonymity, especially in a world where very little email is still transactional relatively speaking.

I do not give (almost) any website an address I send normal email from, nor do I want to. And my default configured client for mailto:, if I even have one on the device I'm using, is not going to be set to the single use mail I want to sign up with.