Comcast blocked my site. How do I fix this?

I have an invoicing app that's been online since 2002 and Comcast decided to block it a couple weeks ago. I have no idea why. I have no reason to suspect anyone has been using it to spam, and I've had no complaints at all about that. I've tried contacting them but their tech support team tells me since I'm not a customer of theirs they cannot help me. They confirmed it is blocked though, but could not tell me why. One of them suggested I message them on Facebook, so I did. On Facebook I gave them contact info for several of my app's (they asked and the users gave me permission to do that), and they said they'd contact them, but they haven't. I've asked users who've complained to me to contact Comcast and several have. Those who have spoke with them tell me that Comcast's tech support said they'd send the complaint up to there "Advanced Tech Support Team", but nothing has changed so they keep calling me. If there were a user spamming with my app I'd obviously deal with that, but I just don't see that happening. Even the users that have been there for 15 years haven't made even 10,000 invoices so I don't see how any of them could be flagged as spammers. I do know that having the word "Invoice" in the subject can set a spam flag off and send the email to a users "Junk" folder, but that's not what's happening. Anyone have any ideas on how to deal with this?