Cannot send email... ESMTP server temporarily not available message

I am a former xfinity subscriber that kept my email address.  I access my email through Thunderbird and recently I started to get this error message when I attempt to send messages --

An error occurred while sending mail: The mail server sent an incorrect greeting:  resomta-ch2-15v.sys.comcast.net resomta-ch2-15v.sys.comcast.net Excessive failed authentication, ESMTP server temporarily not available, please contact support.

I have ensured that the outgoing email password is correct and I have even deleted the password and resubmitted it which allows the system to send messages for a day or two then returns back to this error message and the email does not send.  What else can I do to prevent this issue?