When people change ISPs or employers, their email addresses change. We want to do everything we can to make sure any email address we show you is valid, so after we find email addresses for someone, we send a message to their mail server to verify that the address is still valid.
When you hover over the circle next to an email address, a tooltip will tell you the status of that email address.
If the receiving server allows us to verify the email address and it comes back as verified, we put a green checkmark next to the address.
If the address isn't valid any more and would result in a bounced email message, we put a red X next to the email address.
If the server doesn't allow us to verify the address, but we believe the address is accurate, we put a gray circle next to the address. These addresses are most likely still valid, but some mail servers don't support the ability to verify addresses in the way we do it.