the message seems to indicate that your SMTP settings for authentication are incorrect. all I can suggest is that you post a screenshot of the SMTP server settings pane so some suggestions might be offered.

Here are my outgoing server settings in Thunderbird...

Description: Comcast Server Type: SMTP Outgoing Server Server Name: smtp.comcast.net Port: 465 User Name: [email]@comcast.net Authentication Method: OAuth2 Connection Security: SS/TLS

try port 587 and STARTTLS

Didn't work...connection timed out and didn't send. Put it back to Port 465 and SS/TLS and wonder of wonders, it sent. This is very confusing. Sometimes it will send and sometimes not.

I would suggest trying normal password not oauth. While it does appear in the database for comcast I have a strong suspicion that is is only available for comcast corporate email accounts not customer accounts and it has somehow ended up in the general database for all.

Historically connections were encrypted using SSL. This protocol was fundamentally superseded some 10 years ago by TLS which performs exactly the same function in end to end encrypted connections. Thunderbird still uses the terminology SSL/TLS although the SSL part of the encryption was turned of years ago. Selecting SSL/TLS in Thunderbird is selecting TLS

Outgoing Mail Server Name: smtp.comcast.net Port Number: 587 (SMTP) If Needed: 465 (SMTPS)

These are both provided on the Comcast website in that form, however the accepted standard is to use SMTPS when using encrypted connections i.e. SSL/TLS. So choose port 465.

Encryption: TLS (use SSL if TLS isn't shown.) See https://www.xfinity.com/support/articles/email-client-programs-with-xfinity-email

This discussion from 2019 indicated Xfinity made a change some 6 years ago requiring encrypted connections, so my guess is the oAuth authentication method is not working for you, not the other settings. But check them just to be sure. https://www.xfinity.com/support/articles/update-your-xfinity-email-settings

I changed the authentication to normal password, still on port 465. I am able to send mail now, but for the past few days it would stop sending and I would get a message from the server that there has been "excessive failed authentication, ESMTP server temporarily not available." Comcast tech support was a waste of time. My email works fine if I use their website, and they say that means the problem is on my end. My Outlook had the same problem, which is why I downloaded Thunderbird. This is all very confusing to this old man! Thanks for your response, I appreciate it! Tommy Meisel

do you have an antivirus scanning outgoing mail. Disable that function and see ho it goes. If you have a VPN make sure Thunderbird is exempted from the VPN when connecting to the server. Many US based internet service providers require you to connect to the internet from their network to send mail, (no such requirement to get mail),and a VPN by design actually hides that so they refuse connections.

Check the outgoing server (SMTP) entry in the account settings list of accounts and ensure if you have more than one Comcast outgoing server that the settings for all of them are correct. It is easy to click reply on a mail and have a different outgoing server for that identity selected.

Finally access the Thunderbird password manager and remove all SMTP passwords stored there for Comcast so you can be sure everything is fresh.

It is a support cop out from your ISP that if they are able to keep their website up that any issues you have with mail protocols is your problem. Their stuff is working. That is akin to telling you you have electricity so any issues with your phone are your end. Vaguely related as both use electricity but that is about where it ends. Likewise email protocols for mail clients use different servers and ports to those used by their webmail offering. Both are internet based and that is about where the similarities end.

Thanks Matt for your reply. My Thunderbird has been working fine for the last two days, maybe it has healed...? I do use a VPN, from Malwarebytes, and I checked but if I exempt Thunderbird then it shuts off my VPN period, which makes no sense to me. I appreciate the information you provided. I had previously tried shutting off my anti-virus and VPN, with no improvement. I am still ticked at Comcast for their poor support. Fingers crossed that Thunderbird continues to work OK! Thanks again for your help.

I have endured this as an ongoing issue for months. All has to do with comcast's increasing attention to security. Not surprising Mozilla Support has many questions but no definitive answers, IMHO. Problem is so many variables and custom platforms--am Ubuntu fan, so no antivirus or VPN in the way. Definitely occurs with primary account in Thunderbird with each update of the program. Who wants to wait a few days till it heals itself? Per ChatGPT, best may be to use a 16 character password generated by comcast logging on to xfinity.com through a browser, then Account->Settings->Security-> App Passwords. Generate new string and copy it to use when Tbird prompts for a password.