You could try the LookOut! add-on. https://addons.mozilla.org/en-US/thunderbird/addon/lookout/ It appears not to have been updated recently, but there are recent comments suggesting it is still functional. There are some standalone tools for converting these files, e.g. http://www.winmail-dat.com/ and I think you will be able to find some on-line converters too. You might also want to tell your correspondents that their attachments are of no use to you, and direct them to this article: http://support.microsoft.com/kb/278061