A basic regex is more than enough and you don't even need to deliver a message, just connect to the MX for their domain and check that A) The domain resolves an

A basic regex is more than enough and you don't even need to deliver a message, just connect to the MX for their domain and check that A) The domain resolves an MX and B) That you can handshake for a 250 OK on the rcpt to header only, then drop the socket. Done! It's not that slow and you're leveraging the one thing an SMTP server does really well - be RFC822 compliant. It's something that can be delegated out of process anyway (as a promise or RPC etc) as soon as the email is entered, and resolved when they submit the form. Problem with the email? Then raise it for correction or pass-through... its probably the same amount of code and half the time for end-to-end delivery testing than crafting a bunch of edge case regexes and praying it works.