I would define them by the bar to entry. When a spammer can pick up thousands of domains for a few hundred bucks or less because a dodgy registrar is intentionally selling all domains on a specific TLD for next to nothing I find it safe to block anything on that TLD. I have always done this for email both personally and professionally and never once received any push-back. I explained the risk of collateral damage and leadership was always fine with it. Customers were B2B and could log in to open a ticket. Customer emails would come from domains we already trusted in their on-boarding process.