I don't think I would pay for temp email service, or be bothered to set up open source one by myself. But if I could buy a throwaway domain name from company that specializes in temp emails as a package deal, then I would be tempted.

Right, that actually sounds like a decent idea and since the users would then have verified themselves with their payment details, it would reduce the risk of abuse.

Yup. And I don't think I would care too much about the actual domain, because, well, it would be a throwaway domain. It could be some random characters even. Which means you could possibly buy very cheap domain names in bulk, and re-sell them with low retail price, and profit the difference. The temp email service that you would provide "for free" would be the reason to buy from you.

Got it, this could perhaps work and the downside would however be that the user wouldn't have complete control over the domain, unless DNS control is provided and before I can say the word, I'd be a domain name registrar.

I think it's cool idea though and I might give this a shot, thanks for the idea and if I do implement this then feel free to contact me through the form and I'll set "lifetime" account up for you!