Charter has a history of issues coming, and going. If you continue to have issues it is best to consult their support.

Thank you for the response, Wayne. I did contact Charter/Spectrum and, unfortunately, they said that if I'm able to connect to their website email utility and send/receive messages that way (which I can do), then the problem lies with Thunderbird.

I am able to receive emails through Thunderbird, but when I attempt to send a message, I get the password failure message. It should be the same password for both incoming and outgoing servers.

I hope I don't get stuck in between both companies pointing their fingers at each other.

No guarantee this will help, but open Tools/Options/Security/Passwords/Saved Passwords and Remove any charter.net passwords, for incoming and outgoing servers, then restart TB and enter the correct password when prompted. Also, disable any security/AV program from scanning outgoing mail, if it's enabled. if TB updated around the time this problem began, and your mail-scanning AV became incompatible with the update, it might block sending.

Sorry to say, but this appears to be a problem deeper than I can or have the time and energy to put into it. Both my computer and my wife's computer exhibit the same symptom's: Thunderbird will not send an outgoing message to Charter.net's server. So I downloaded a different email client app and it was up and running in only a couple of minutes. Plus it transferred all older emails, contacts and other information with the click of a couple of buttons. I'll be making the change to my wife's computer, too, unless someone knows of a simple fix for this.

What are your smtp server settings? These have worked for other users:

mobile.charter.net on port 587, SSL/TLS security, authentication = normal password, User Name = email address.

https://support.mozilla.org/en-US/questions/1234783#answer-1157069

Thanks sfhowes, that seems to have done the trick. We've been using smtp.charter.net (as first instructed by Charter) for years. Not sure why all of a sudden that stopped, but changing it to mobile with the different security settings got it working again.

We don't connect with a mobile device, so that seems rather counter-intuitive, and I hope that won't mess up anything else in the long run. But so far sending and receiving emails has been working just fine.

I have same irritating messages but it seems to work anyway, I have the correct settings since I can send and receive mail, if I change to charter address it rejects my password.