Hi, are you getting a specific error message? When I searched for AT&T's instructions, I found the following, but I don't know if this is the latest information: * Servers: https://www.att.com/support/article/dsl-high-speed/KM1010523/ * Secure key: https://www.att.com/support/article/email-support/KM1240308 ''Also, a moderator can move this question to the Thunderbird forum.''

More/Related Info: • [https://support.mozilla.org/en-US/questions/1315734#answer-1373022 AT&T , ATT-Yahoo related settings for TB]. • [https://support.mozilla.org/en-US/questions/1304518 Yahoo , ATT-Yahoo related settings for TB]. • [https://stackoverflow.com/a/63255601/3553808 Access WebMail Type Of Email-WebSites From TB-WebBrowser-Tab], or, Configure TB to allow very specific OAuth2 based website-addresses in TB's Cookie-Exception list, etc, etc. ''(please upvote my Q+A both, inside the linked page, if its useful/helpful)''. • [https://stackoverflow.com/a/64980413/3553808 Disable auto-update or update of TB] ''(please upvote my Q+A both, inside the linked page, if its useful/helpful)''. • [https://support.mozilla.org/en-US/questions/1315734#answer-1373915 Disable AV/FW/SS software] mail-protection OR enable+add+allow EXCEPTIONS in it & add mail-server names/address . Some AV/SS creates insecure proxy/gateway for scanning/checking emails, but there are other secure way to scan/check emails . Please make sure the option "Allow AV to Quarantine email" is selected inside TB, and make sure your AV/SS is set to "Scan all files on access".