Can't check email from AT&T network

This has happened before and it is now happening again.   I cannot check email from an AT&T network.   Email works fine from a Verizon or Comcast network.   Device does not matter... does not work from iPhone or  PC.

iPhone gives the message:  The mail server imap.comcast.net is not responding.  Verify that you entered the correct account info in mail settings.   Server code 'ALERT' server message:  Temporarily blacklisted IP Address, try again later"

Settings are fine... it works from other networks.  Client does not matter.   It fails on all clients when using AT&T networks.  I can access the email through the Xfinity website just not through Outlook/Thunderbird/Apple Email clients etc... 

It has been week or so now.   

This is a recurring problem.   It seems to align when a router gets a new 'temporary' IP from AT&T residential fiber service.   There is nothing nefarious going on... just the occasional new IP from rebooted systems and the fact that it only seems to happen with AT&T networks...