So the tech came out and I sat with him. He didn't have any clue about the IP configuration (Comcast didn't give him that information!) and he showed me how to log into the cable modem and I took the controls from there. You see, it's that anal retentiveness to write things down like configuration data and save it for years! Years, I say!
Ahem.
Anyway, after about 10 minutes, everything was back up and running as it should be. The drive light on the mail server has been almost constantly red, meaning lots of mail is finally flooding through.