The best way of verifying an email

[eluser]stanleyxu[/eluser]
Okay, your function is good enough. I think, a DNS check is too much and too strict... (I am not very sure, if a slow DNS check would block script executions of other requests.)

Actually, for anti-spam propose, you can simply check the syntax of email and let user enter some security code (Captcha) or questions.

If (valid) fake mail addresses are not welcome, you should disable comment for guest.