SORBS delisting

According to Mxtoolbox, our static gateway IP address is blacklisted on SORBS Web and SPAM. We did some cleanup some time ago to fix access issues to our internal SMTP relay that I suspect were the cause of the original listing.

According to SORBS, however, it seems that any de-listing request needs to come from the IP address owner, which is Comcast, or it can somehow be delegated for certain IP addresses. How do we go about getting our IP address de-listed?