Outgoing Emails Won't SEND On My Thunderbird Client

After years of using Thunderbird to receive and send emails, all of the sudden my emails will not send.  When I try I get this message:

Sending of the message failed. An error occurred while sending mail: Outgoing server (SMTP) error. The server responded: resomta-a2p-647141.sys.comcast.net resomta-a2p-647141.sys.comcast.net ESMTP server not available (396356/151.247.189.217).

I don't know what this message means since I test the server and it says it's okay: 

Description: Comcast Server Type: SMTP Outgoing Server Server Name: smtp.comcast.net Port: 465 User Name: [EDITED: Personal Information] Authentication method: OAuth2 Connection Security: SSL/TLS

When I test the server, I get this message: Successfully established a secure connection to smtp.comcast.net:465.  This tells me the port is good.  I don't know what the ESMTP is.  What's wrong??