Hey!
I have been searching for them for a long time too. I have an email list of close to 10 mil which is quite old and figuring out the best method to clean them up.
The manual method that I could come up with is something like this:
1) Use an email verifier software to verify email addresses. Make sure that you use proxies
2) Once the emails are verified, I send out a welcome message or a message similar to " Hi, we have your email address in our database. We thank you and will ensure you receive quality emails". Something similar
3) Put in an unsubscribe button at the end, just to ensure that you are complying with the CAN SPAM.
I do know that this is a long and tedious job, but, considering that an email can get more money I go for it.
Dont know if this really helps out, but just sent an idea
P.S If you do come across a software anything, please let me know.