what is the server name you are trying to send through and the domain of the email address you are using. My guess is you are trying to send mail from say me@work.com and sending trough a mail server like me@home.com.... other instances of this error indicate a change of policy at TWC. What provider are you using?

Matt, thanks for your help. I suspect you are right about the change of policy due to the purchase of TWC by Spectrum. I hadn't thought of that! The message pops up after sending a high volume of newsletters to my contacts. Spectrum probably has a lower daily limit. It's disappointing that the Spectrum tech support didn't inform me when I told them of the error message. I have a feel for the new limit level and will just stay below it next quarter when I send the next newsletter. Hopefully, I never see the error message again.

Since the providers server issued the error message I wonder how their help desk denied it is their problem and why you believed them.