You can verify whether the syntax of an email address is valid with Email::Valid (and friends).

But to verify whether it's semantically valid can be done only one way... send email to the address with a unique token, and have them return it (in a way that's not a bounce message) or visit a web site.

There is no real-time way to do that. You'll get both false positives and false negatives. As a specific example, there is no way you can verify that jeffrey.dahmer@stonehenge.com is a valid email address in real time. You have to just send it. And you'll either get a bounce later, or a response.

I have an example of doing that for mod_perl, and an older version for CGI.

-- Randal L. Schwartz, Perl hacker
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