How do I check if an email address is valid without sending anything to it?

Question

I have a client with 5000 emails from an old list he has that he wants to promote his services to. He wants to know which emails on the list are still valid. I want to check

Answer 1

I think you need to send the emails to find out. Also, this is pretty much exactly what a spammer is, thus the reason for getting put on spammer lists. Sending in bursts will help you hide this fact though.