I finally found a solution that worked for me. Search and delete Keychain entries for mail.charter.net, mobile.charter.net, and smtp.charter.net. Then re-add your Charter email as a new account from Mail preferences. Many months ago I tried the suggested community solution "delete keychain entries for your email". This never worked for me because it was too general. I had a large number of Keychain entries for websites where my username was my email address. Therefore, I never selected the correct Keychain entries to delete. I finally had the idea to search not for my email address but for the server names. I hope this post will help other Charter users who continue to experience Mail problems.