I really need your help. Here are the particulars requested for helping to troubleshoot the problem: Email provider: Compuserve Mail, powered by AOL Mail ISP: Comcast Firewall: Windows Defender Antivirus Software: Windows Defender OS: Windows 10, 20H2 Thunderbird version: 78.10.0 Like others I cannot send outgoing mail, I've read several AOL & Mozilla & Thunderbird articles, questions/responses on support site and tried the steps provided at http://mzl.la/1ApHITV. However, my problem is a little different: After trying to send the message I get a popup Yahoo sign in page AND the following Thunderbird error message: "Sending of message failed. The message could not be sent because connection to outgoing server (smtp) smpt.aol.com was lost in the middle of the transaction. Try again." I tried again and same result. I have uninstalled Thunderbird, deleted and then added email account, installed & reset the incoming and outgoing mail servers using the appropriate server settings, turned off firewall and antivirus software. For each attempt, I would close Thunderbird and reopen and try to send an email to myself. Same issue. Another problem is I do not have a Yahoo account so the sign-in page does not recognize my email address and will not let me proceed to entering the account password. I receive incoming mail just fine, using the OAuth2 authentication method. When prompted by AOL for email address, it is recognized, as is the account's password. Also, I was successful in sending messages through the email web browser, so I know this is not a Compuserve or AOL issue. I've used Thunderbird for years and did not have this problem until the app was just recently upgraded to the latest version. Attached is screen shot of the Outgoing Server Settings. You help is most appreciated.