I had the same issue and it did take quite a bit of digging to nail down. Comcast Business with 5 static IPs, same setup as yours.
1. Make sure your reverse DNS entries are correctly configured such that the domain of your reverse DNS lookup will match the domain your messages are claiming to be from. dashed-ip.sea.wa.comcast.net will generate spam warnings on many mail servers if your server claims to be mail.joecorp.com. Call Support and they will update it for you on the phone within a couple of minutes. Also make sure you're not in a residential IP block.
2. Make sure you're not actually an open relay or otherwise allowing unauthenticated senders to generate outbound messages. I was using MailEnable, and had it misconfigured such that it wasn't actually doing the authentication I had selected. This got me blacklisted quickly. A few bounce messages had links to the blacklists themselves to submit appeals; they'd dutifully take me off each time but I'd get re-added automatically. It took a few weeks of trial and error to get this one fixed. I know you say you're not...and I thought I wasn't either, having specifically taken steps to disable open relaying. But it turns out I didn't quite get it the first time, and was still relaying messages without authentication.
I'd imagine issue (1) may be a big contributor to your problems, personally.