It sounds like the name of smtp server has changed. I suggest you check with your email host for proper settings. If you post server setup here, someone may know the proper setting, but your email host is also a place to check.

Thanks for the suggestion. I contacted Comcast and verified the settings ... SMTP server and port number.. and they are correct. I can send outgoing mail from Comcast mail site, but not when using Thunderbird.  ?????

Can you post a screenshot of smtp server settings and also one of the error message? There's something we aren't seeing.

ending of the message failed. An error occurred while sending mail: Outgoing server (SMTP) error. The server responded: resomta-c1p-023412.sys.comcast.net resomta-c1p-023412.sys.comcast.net Excessive failed authentication, ESMTP server temporarily not available.ending of the message failed. An error occurred while sending mail: Outgoing server (SMTP) error. The server responded: resomta-c1p-023412.sys.comcast.net resomta-c1p-023412.sys.comcast.net Excessive failed authentication, ESMTP server temporarily not available. smtp.comcast.net port 465