I have an email address listed as a hard bounce, but I know it is valid. What happened?

  If an email was blocked by a spam filter on the recipient's end, it could cause it to come back as a hard bounce. This spam filter could be in the company firewall, the ISP/data center, or the subscriber's own computer. A valid email address that has been marked as a "hard bounce" can be removed from your contact list and re-added to erase the history of bouncing. If