Can't send outbound mail under Windows 8

I have used TB on many platforms, up to Win 7. I have a new Win 8 laptop. I have installed TB 16.0.2. I can receive mail, but I cannot send mail. The error message is "password is rejected" or some such. I can use this same log in at RoadRunner and log in fine, but I can't send mail using the RoadRunner outgoing mail server with Win 8. Note that that this same sendmail process works great on all other machines.
The extremely unhelpful RoadRunner chat analyst finally told me that Win 8 doesn't support Pop3...could that be true, or is it true only of some mail app on Win 8? We have installed TB as a separate app, so I have trouble believing that POP3 is totally prohibited on Win 8.
Anyway, I have looked here and around the 'Net, and have found only questions on how to make this work, but not answers...thanks!

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