I've found a couple online services that do this, and I found this post at stackoverflow about it:
How to check if an email address exists without sending an email?
The problem is that the PHP script linked to there requires you to populate a list of nameservers and domains, and thus (I think) only works if you are validating emails on a known domain. I want something that will work for any email (at least work with a high probability). I found a script that does it that I can buy for $40, but I'd rather find the same thing as open source.
Thanks for any advice, Jonah