I notice security companies such as Rapid 7 prevent people signing up with either free email accounts, or accounts provided by ISPs:
To prevent software license abuse, this field requires an email address from a domain you or your employer owns, such as a company, university, or personal domain. Email addresses from internet service providers or free mail providers are not accepted
Now I imagine that this could be accomplished by a blacklist of ISPs and free email address providers but I also notice that it does not work with some of the temporary email address services so these must be catered for somehow. My other guess was that it did some sort of DNS lookup to validate the company, but this would not work if I had set up a domain using my own company solely for temporary email addresses that the general public could use.
My question is how does the form validation work to ensure that your email is not any form of temporary address?
The above page validates the email on lost focus, so you do not have to actually submit the whole form to test this (as I would not advocate spam).