About twice a quarter I get an email from a client worried about failed email due to block lists, specifically regarding AT&T. AT&T's email system seems to have an aggressive black listing policy and uses non-reputable black lists in its system. Specificly UCEProtect lists Who are hyper aggressive block entire IP ranges and then extort you to get off the list.

So twice a quarter I have to reassure my client that it is something on AT&T side. We are not blacklisted on anything other than Russian based or non-reputable black lists.