Temporarily Blacklisted IP - Try again later

I am using Thunderbird and Apple mail. Both of these have been working for years. Starting a couple of months ago I have been getting the message "Temporarily Blacklisted IP - Try again later" when checking for mail in Thunderbird. This problem is intermittent. Checking mail works fine for a while then I get the message again. A few hours to a day or so later the message goes away and I can receive mail again.  The same issues occurs with my iPhone when the phone is on my WiFi. When the phone is on cellular data I receive messages correctly. 

How do I resolve this issue?

Did something change on your end?