Personally, I would delete all email addresses who haven't logged in since 2009-12-31 (I would probably increase that to 2011-12-31). We don't know how many email addresses are we talking about here. But I would seriously suggest if they haven't logged in for 2.5yrs, they are unlikely ever to log in again. So, you delete one or two valid email addresses. If they come back to your forum, they can always register again. Not a biggie, imho.
I have had some success with Advanced Login System (ALS) found on ThemeForest.net. I have identified hundreds of dead email addresses with this software, without ever getting my IP's or domains banned. YMMV.
But if you are serious about this cleaning, you could use MailChimp and stay within their free limits, by adding 2K emails send them an email, eliminate the dead email addresses, save live email addresses, delete all email addresses on MailChimp, rinse and repeat. You could do this 6 times before reaching their 12k emails per month for the free account.