G-lock Software's Advanced E-Mail Verifier will do that. They offer free and reasonably priced personal and business editions of this package.
Info link:
http://www.glocksoft.com/amlv/I don't know if it has a throttle control. But you could break you list into smaller chunks and run them separately if you're worried about pissing your ISP off by looking like you're a spammer. Be forewarned, many ISPs are onto the "slow list" ploy so now they also monitor packet type trending. So doing back to back address "verf" requests may trigger a security alert if your list is very large. When in doubt, let your ISP know what you want do before you do it. It's a pain, but it's still better than getting shut off if they misinterpret your actions.
Just my tuppence.
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Disclaimer: 40hz is not affiliated with G-Lock Software, nor does he get anything for recommending their software. He buys his licenses from them just like everybody else.
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