Why can I not send bulk emails using comcast webmail or smartzone?

I am trying to send an email to all the people in my address book using Comcast.net and there smartzone. It keeps coming back telling me there is a problem with an email address. These email address are ones I have been emailing for years and they have not changed. Your help or knowledge base says to clean out history and cookies. Which I have done. Restarted the computer, still same problem. Right now I am very un satisfied with comcast.net and there smartzone webmail. Is there anyone at comcast that can help without telling me it is my computer or I need to cleanout my cookies?