Emails cannot be sent. Can anyone help?

Question

I have a total of four email addresses that go through Warner's RoadRunner service. Three use the rr.com address and one uses the mac.com address. The mac.com address will receive AND SEND emails but the rr.com addresses will only receive. I cannot send emails using the rr.com addresses. They have worked just fine for years when, all of a sudden, they stopped sending for no apparent reason.
I called the RoadRunner techs and they took me through all of the "preferences" selections and everything checked out just fine on that end. They then took me to the RoadRunner web site where I was able to both send and receive emails with all of my addresses. At that point in time they told me that the problem was not with RoadRunner but with Apple's mail.
I have read the discussions and tried the usual fixes: I've pulled and rebooted both my RoadRunner modem and my router. I've deleted "envelope index" from LIBRARY/MAIL (which usually works, by the way, for most of the problems that I have ever had with mail) but nothing allows me to send mail again.
Any assistance/solutions would be greatly appreciated.
doug o.

Answer 1

Doug,
The reason the redundant authentication fails, I think, is due the response the SMTP server gives (part of the handshake with the server). If the response is unexpected, then Mail simply quits trying. A guess is that the response got changed by rr, and as is often the case, the redundant authentication attempt caused a failure. Although open to you, at your home venue, based upon trusting you because of the IP address of your modem, rr probably has more tightly shut the door to other SMTP. The old setup was sort of "grandfathered in" but no longer after you upgraded the OSX (some dispute the grandfathering theory, but it is all that explains some reports).
Ernie