If you're asking for an email address in exchange for some piece of content (e.g. a soft paywall), I find that it's reasonably sensible to filter out the absolu

If you're asking for an email address in exchange for some piece of content (e.g. a soft paywall), I find that it's reasonably sensible to filter out the absolute spammiest-looking of email addresses in order to collect more real email addresses and prevent people just rolling their face across the keyboard and calling it a day. (It can also help with bots a little bit, although most bots are smart enough to use <random firstname>.<random lastname>@gmail.com or something). The best way to do this is obviously double opt-in, i.e. actually emailing the address in question. Most businesses believe (probably accurately; I don't have the stats) that this creates too much friction. I've used Sendgrid's email verification service in the past, and the actual scoring is basically garbage but if you set the threshold really low (e.g. reject all emails with a score between 0 and 0.1) then your precision is really good (almost every email address you reject is invalid, as measured by actually sending an email to them and the email hard-bouncing or being otherwise undeliverable). You let a decent number of bad emails through but it's an improvement over nothing.