Please post the settings for the account that the other email programs are using. That may help. Screenshots of actual settings are preferred.

Attached is the server settings from my GMX mail account. I cannot get my Charter/Spectrum server to reveal any information about my pop3 four email accounts I've had with them for years. To set up an account with Charter/Spectrum they now only give instructions to set up an IMAP account. Did some research and the second attachment does show Charter SMTP settings. Info is not from Charter/Spectrum and may be outdated. Those are the pop3 settings when I configured my email addresses with Charter. Now I can't send only receive email with Thunderbird.

This site has different settings and may help: https://ispsettings.co/charter-spectrum/

Just solved the problem with Thunderbird receiving but not sending messages with multiple accounts with Charter mail server: Goto TOOLS>Settings>Account Settings> select Outgoing Server (SMTP) Click ADD In Server Name enter: smtp.charter.net Change PORT to 25 Enter FULL email address in User Name (my accounts only had the user name not full address) Click OK Select the account just added and click set DEFAULT. CLOSE Open Thunderbird and send a message from the address just setup. Do the above procedure for each address with Charter. This worked for me.