One of our email accounts is having the winmail.dat problem. All of the settings in their outlook have the HTML setting checked so nothing is going out as Rich Text Format. This only happens to some of their email recipients.
I send the same files to test this problem from other godaddy email accounts and it does not have the problems that the other email account does.
Even though these settings are checked correctly is it still possible Outlook will send a file not as HTML thus causing the problem?