those three digit error codes originate with your mail providers SMTP mail server. You do not mention your provider, so only generic information can be offered.

Note this often means you have sent more emails that your providers terms of use allowing in a given time. Hence it being intermittent probably.

Thanks, Matt, for your reply. My provider is RCN. I engaged in a long chat discussion with a customer support person at RCN after receiving your response. They disavowed any responsibility for the problem I am having by saying that they can't help me if I use an email client other than RCN's own web mail. The agent obviously didn't recognize or understand the error message. I pointed out three times that the error message points to a problem with RCN's email server, as you also suggest, but got nowhere. I remain stuck. I did change my email password, which was a suggestion from a different blog, but the problem remains. Any additional thoughts would be most welcome!

I should have noted that I have not been sending an unusually large number of email messages recently.