This is really just a problem for spammers going out and either buying mailing lists that haven't been validated or scraping the web for email addresses. In the

This is really just a problem for spammers going out and either buying mailing lists that haven't been validated or scraping the web for email addresses. In the case of the spammer, they would probably care a lot more about their bounce rate than their false negative rate (i.e. valid addresses that fail some sort of validation regex). In fact, they would probably tune their validation to actually throw away addresses that didn't look correct just to be safe. Obviously, this is a different scenario than your bank not accepting your valid (per RFC) email address. Which is why any sort of blanket advice is pretty dumb. Not that I care to aid spammers... The other scenario might be a site that puts up a "paywall" type thing, where you are forced to enter an email address to gain quick access to something, but doesn't want to bother you with going and verifying an email (e.g. instant discounts, downloading a PDF, etc.). Or in-person email address collection when you buy something in a store. It's never a good idea to collect email addresses of people that have no desire to subscribe to your marketing.