I wasn’t aware of Comcast being able to shutoff ports for just a single user. They can block the ports being used from a specific IP however… which leads me to believe that you (your router/computer/etc) recently acquired a new IP address that the previous owner used to spam emails out through.
Therefore my first suggestion would be, before going through any other hassles: Try renewing your connection to get a new IP. There are several ways to do this…
If you own a router, you can either try shutting it down for ~30mins-1hr or change its external MAC Address (if your router provides for it).
Otherwise, if you’re using a cable modem/dsl modem, unplug it for ~30mins-1hr. That should give you a fresh IP.
Just keep trying until it does if not.