If you are using googles popular mail services GMail over IMAP and demand on winmail.dat files you will run into a problem.

GMail IMAP doesn’t return the senders original winmail.dat file, it return a home-cooked file which have only the name in common. In this case, not even Microsoft Outlook can handle the winmail.dat file.

I tried to contact google about this issue over many different ways, but didn’t get a answer.

There are two workarounds: Switch back to GMail POP3 or use GMail web interface to access mails with winmail.dat attachment.