Over the past few years, I've noticed there's a bunch of email validation companies that will constantly hit your mx to see if there's a catch-all, or if a particular address is live. These don't seem to show up in most consumer email systems - hotmail, gmail, yahoo, proton, etc. but certainly show up in my domain email. I'm not sure if this is because there's less trust implied with my domain, or because the big guys silently allow them but don't pass them. Regardless, this is what I've blocked so far: mails.so businesspagesonline.com bouncer.cloud thebouncer.io truelist.io validator.com applicationpages.com example.com emaillisttracking.com mail-scraper.com ipqualityscore.com emailage.com pretty much anything with .io as a TLD It does make a few things difficult, for example credit card companies do use these to check your email address, and cellular carriers do it to. But there's no need for them to CONSTANTLY do it, and fuck you, you're blocked. Too bad.