This is an appalling situation whereby an anti-spam device manufacturer forces you to register your domain ($20 administration fee per domain!) in order to ensure your domain isn't 'blocked' by companies using their devices.
To put into context, I have a small business which provides software services to companies. I rely on my email system to be able to communicate with everyone. Recently I started to received SMTP problem reports from customers who weren't receiving my emails. It turns out their anti-spam system (Barracuda) was blocking me. I checked mxtoolbox to find out what the issue was and discovered only 1 of the 50 companies who provide spam blacklisting services was blocking me - Barracuda. I went to the Barracuda site and it said it couldn't give me an exact reason why I was being blocked (it listed several possible causes), but I immediately smelled a rat because none of the other reputable spam blocking sites had me listed and I would have thought they would all use at least similar techniques to determine if I was a spammer.