IP Address Blacklisted - Email Won't Sync

Hope someone can help. We have been having an intermittent issue with Comcast emails that we thought had been solved but it's back.  We started receiving various error messages about the inability to connect for emails for 3 different accounts on 5 different devices. The most specific messages says "The server for the account (our email) returned the error [Alert Temporarily blacklisted IP address - try again later]" followed by directions to check our passwords.

After much troubleshooting on our part, we isolated the issue exclusively to our wireless network in our home, which is on Optimum.  Emails can be sent and received on cellular on our phones and ipad, as well as a laptop when it is hardwired to our router. Their help desk was perplexed and suggested that we get a new router, which we did, and the problem disappeared.  Until today, when it came back.  We are at our wit's end! It is clearly a very specific issue between our Optimum wifi and our Comcast email server, but neither company feels they own this issue. To be specific, Comcast washed their hands of our problem because we can get mail on their website and Optimum says they can't help because we have wifi, it's just the email server that's a problem.  Help! Can anyone here offer any suggestions?