I worked in a lead-generation agency for some time. We were doing competitions that could produce 50-100K entries per day, where the end result was a marketing

I worked in a lead-generation agency for some time. We were doing competitions that could produce 50-100K entries per day, where the end result was a marketing email encouraging you to buy something. So we did lots of multivariate testing, with permutations of regex patterns, MX record validation, sending a confirmation email, client-side only, server-side only, etc. What we found was that a decent regex pattern (we used \b[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,4}\b, taken from http://www.regular-expressions.info/email.html http://www.regular-expressions.info/email.html), with MX record validation and common junk domains blocked (e.g. mailinator.com) produced the largest conversion rates in the follow-up email. In other words, less validation produced more entries, but they would have been lower quality, which affected our sender reputation and cost more. The confirmation email was awful for conversion. YMMV.