ESMTP server temporaily not available

This was asked about 3 years ago but there was no resolution to the problem.  In the last couple of day, I have been getting the following error when I try to send email from Thunderbird:

An error occurred while sending mail: Outgoing server (SMTP) error.  The server responded: resomta-h2p-555030.sys.comcast.net resomta-h2p-555030.sys.comcast.net.  Excessive failed authentication, ESMTP server temporarily not available. 

I was on the phone with tech support but he could not find the issue.  My settings for the outgoing server (SMTP) settings for all of my comcast accounts is:

Server Name: smtp.comcast.net

Port: 587

User Name: <my full email address>

Authentication method: Normal Password

Connection Security: STARTTLS

I have also tried SSL/TLS connection security on port 465.

Does anyone know why this would have started in the last couple of days?

Just before this started I sent an email and didn't know how big the attachment was until I sent it.  It was right near the 25 MB limit.  Is that e-mail stuck trying to send?  That would kind of explain the "excessive failed authentication" part.

I can receive e-mail fine.

When I try to create the account in Outlook, it just says there is an error -- not specific.