Meaning that even if you transmit e-mail in good faith, you can't force others to accept it. I just wish that mail administrators understood that. More than once my mail server has been rejected by an institution that relies on a 3rd-party blocklist, and that mail administrator then tries to tell me it's my fault and I should take steps to remove my server from the blocklist. Of course, it's his own fault for subscribing to a blocklist that incorrectly blocks mail from my server. #