Well, my short take's likely clear from past posts on this: I'm not persuaded that prohibiting what are often erroneously called "personal" addresses is useful. Our clients include enterprise software houses, B2B financial services, high-end B2B consultancies -- and none of them block large webmail domains, since that's not enough evidence that someone is a weak lead.
My longer take:
As an enterprise software buyer myself, I often do software evals with my Gmail account. This protects my own privacy and my clients' privacy and doesn't make my interest any less serious. If someone demands a "business" address, I of course understand why they're doing it, but I also, to put it informally, think they're lame. Might still fill out the form with my own business address if the product is otherwise promising, but then they'll just fetch firmographics for our consulting firm, which is no more relevant to the deal than Alphabet, Inc.'s financials!
Suggesting that the lead supply a business address, IMO, is reasonable, if the copy shows you get it -- something like "We know you don't want us tracking you down at the office. But to make sure our emails don't get lost among your weekend plans and forwarded memes, please provide your business address."
On the SMB side, millions of people operate real businesses from free webmail addresses. We may wish they didn't, but it's silly to turn away locallawfirmwithlotsofcash@gmail.com or highnetworthfinancialadviser@yahoo.com.
Also, disposable addresses are different, and I've contributed JS that shows how to use centrally managed freemail-disposable list to block those domains. Such leads are likely to be trying to get your content and never be contacted again. (Naturally, a secondary Gmail address can be used for the same purpose, but you'll never know.)
But in the end, it's going to be a case-by-case thing. The only way you can know for (relatively) sure is if you allow the leads into your company's db and do some thorough metrics later. Get one 100MM deal from an address @aol.com -- this has happened at a major medical tech company we work for -- and that'll probably be enough. Absolutely zero wins from webmail over the course of a year or two can be a good reason to try blocking. Yet you also should have some way of finding those people that tried with Gmail and then changed their address as required (you can add a hidden field to the form for this) and a way of finding abandoned forms (this would have to be via GA or other anonymous analytics).
Finally, remember that if you block known webmail domains on your form, yet still let people download your content without at least validating that the mailbox exists, you're leaving a gap. What's the bigger problem at your company: leads that can't be contacted at all, leads that receive mail but never engage, or leads with webmail addresses?
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