Like I said yesterday, if I wait awhile, the problem seems to 'self correct'. When I got up this morning, I could again send emails. I sent several, including group recipients with an attachment, from both my old and new computers, and it seems to be working ok. I have not made any changes to the settings or any other adjustments to my computers since last night when it was not working.
My ISP is Time Warner Cable. I specifically asked if their server was recognizing my emails as spam, or if I exceeded my limit. They said 'no', and that my account was not flagged for quarantine. They could not shed any light on that error message.
"Looking at the error you are getting, "server error: '451 Outbound mail refused", it would seem that the email is leaving your computer correctly, but being refused by the email server."
The reason I hypothesized that the message was not even leaving the computer is that as soon as I would send a test email, it would come back immediately with the 451 error message, I mean instantaneously -- there's not enough time lapse for it to get to the server. Having said that, I've always thought Time Warner shuffles settings and things around for its customers then plays dumb like nothing has changed. I'd still like to figure out the likely cause of the problem in case it happens again, and still want to rule out Norton. But arrows are pointing to ISP.