You said mail is bouncing back, but you did not indicate where it bounces from. Is this incoming mail to your server that bounces back to sender? Or, is this mail sent from your WHB server that bounces back to you because of the blacklist?
If it is the latter, then the first step is to report this to Support. They will investigate to find the source of the problem on the server, then they will contact the blacklists to request removal.
Unfortunately, blacklisting happens periodically for shared servers. Likely someone on your server was sending spam, or had a high-volume mailer sending uncontrolled amounts of mail in a short time. Because all senders on a shared server share the same mail server IP, when that spammer gets blacklisted so does everyone else on the server.
In the meantime, I suggest that you tell your clients to set their ISP's SMTP server for outgoing mail, thus bypassing any blacklist issues with the WHB server. IMHO, it's better to have your clients always use their ISP's SMTP server for outgoing mail.
I am not WHB staff and I am not paid.
I provide help in these forums on my own time.