we had a similar problem a few months ago except we were being Joe Jobbed, with 30% of the mails coming from verifiable Comcast addresses.
In the end we had around 200-300K emails and our external mail system was crawling for a few days.
So far Comcast have yet to reply to our requests.

All it means for us is that next time it happens, we just drop anything from the comcast subnets.