You will probably run into a few issues. First you need to fetch the MX
record for the domain before you can even connect. After that you could use
the SMTP verify user command to see if the user exists,
...or just use Email::Valid module.
but I think you might find that many SMTP servers will not give you a reliable answer. I think AOL, for example, will always tell you that the user exists (I may be wrong though).
Basicaly you cannot know for sure if any given email address is valid. You'd have to send an actual email, wait some time for bounces, and even then you wouldn't be sure. This is why websites requiring email address often send password in email during the registration (which is completely insecure but makes sure the email is valid).
I think you might be better off with Spam Assassin, or using an ISP that uses it.
I agree.
On the other hand, it can't hurt to try doing it yourself.
If we don't count the DOSing and lots of false positives then of course it can't hurt.
-- ZSDC Perl and Systems Security Consulting
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