I've not used a single website that hasn't flooded me with "email newsletters", "promotions" and "reminders". Even my damn utility companies (each on their unique addresses) both sold and spammed the address I gave them with the stuff.
There is no trust.
It might be anachronistic, but if a potential customer wants to begin a relationship with us with a lie, then of the two of us, I would think we have more reason to mistrust that individual than they have to mistrust us. Their mistrust is based on projecting bad behavior of other companies onto us whereas our mistrust is based on them actively beginning the relationship with a lie.
Of course, this decision may cost us some potential customers, but for now, that's something that we're willing to accept.
Solid justification for disallowing them. My service doesn't even send a reply-verification to users (but Free users get a CAPTCHA... sadly) and just assumes validity. Perhaps I ought to integrate it at some point, but personally I hate having to click to verify my email address. Comes back to trust, again.
If they are serious about evaluating our product, they can provide a real email address. If they aren't very serious or inherently don't trust us, then I'm willing to miss the opportunity of having them as a customer.
I respect their privacy, because we don't spam, sell, or abuse any of these email addresses ever, but I find it hard to trust anyone with an account on our service if they use a fake email address. Personally, when I sign up for services, I find it helpful to gauge the company based on how they use my email. If they automatically start sending me marketing materials the next day, that tells me a lot, and I'll generally cancel the service and report all of the subsequent marketing emails from them as spam. The only way to do that effectively is if I use a real email address.
Do people use these fake inboxes for any reason other than trying to prevent or cut down on spam? Am I overlooking some key aspect of allowing people to use these email addresses?