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?