Error message, can't send e-mail

I've had my Comcast email for probably 20 years and have been using Thunderbird forever.  Every other day I'm not allowed to send e-mail and I get the following message. I've changed all of my passwords many times.  I have got to get this fixed or figure out another solution.  I need to be able to send e-mails again.  I'm not terribly good at this and need solution ideas in very simple terms and steps.  Thank you a million if you can help me!!!!  

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.