Our email is blacklisted and we need it fixed!

Our main domain name is on the blacklist for Gmail, Hotmail, Yahoo and others. We need to get our domain(s) removed from these lists. We are not a mass marketing firm and do not send any spam to customers. If you have experience with removing email from these types of lists, we would greatly appreciate the help. Our preference would be a step-by-step process so we can accomplish this ourselves, either through writing or over the phone with screen sharing. The domain that is in question is www.thecummingsgroup.com. It is the primary domain, so everything under this domain is also being affected. Thank you