Sending of the message failed. An error occurred while sending mail: Outgoing server (SMTP) error. The server responded: resomta-c2p-555662.sys.comcast.net Excessive failed authentication, ESMTP server temporarily not available.

- What did you change the port from, what to, and why? - How long has this been going on? It's difficult to tell from this error message exactly what the problem is. It implies two things: - You have tried to authenticate too many time with a wrong password or OAuth2 token. This could happen if you have multiple devices checking or sending email, and one or more is sending the wrong password or is not OAuth2 authenticated. Have you changed your password recently? If you have been blocked for excessive wrong passwords, that might trigger the next part of the error message: - Their SMTP (sending) server is truly not available, that they are having technical problems on their end. OR they have blocked you. Difficulty with connection can also be due to local anti-virus/network security/VPN software. If you use any of those, try temporarily disabling them or creating an exception for Thunderbird. Here are my working Comcast SMTP settings. How do they compare with yours? IMAP (Incoming): Incoming Server Type: IMAP Mail Server Incoming Server Name: imap.comcast.net Port: 993 User Name: <my email address> Connection Security: SSL/TLS Authentication method: OAuth2 SMTP (Outgoing): SMTP Server Name: smtp.comcast.net Port: 465 Connection Security: SSL/TLS Authentication method: OAuth2 User Name: <my email address> If you are set to authenticate by OAuth2, your authentication token may have expired. You can force a re-authentication by the following method: - Make sure cookies are allowed (see <Settings | Privacy & Security | Web Content | Accept cookies from sites>); - Delete all cookies for that account (see <Settings | Privacy & Security | Web Content | Show Cookies>); - Delete all entries for that account from saved passwords (see <Settings | Privacy & Security | Passwords | Saved Passwords>); Now restart Thunderbird. You will be asked for your credentials, an OAuth2 token will be set, and hopefully they will be accepted and mail transaction will resume.