Mozilla is the organization that is responsible for the Firefox browser. It is also responsible for a product called Thunderbird, which is a very good email client which support both POP and IMAP.
One draw back of Thunderbird is that it does not support the opening of TNEF formatted mail. TNEF is the format generated by MS-Outlook when the user composes a message in Rich Text Format (RTF) — which is a common occurrence.
It seems that the Mozilla Thunderbird developers have know about this deficiency for quite a long time but have decided ignore the issue because TNEF, despite being a defacto standard due to the large number of outlook users, is a proprietary format.
This is clearly a situation where the Mozilla organization has chosen to curse the darkness rather than light a candle.
Fortunately, Aron Rubin has developed an add-on that will address this issue. It works well and it appears that Aron will continue to maintain it. So if you use Thunderbird and are tired of receiving windmail.dat files please give his add-on a try.
Hopefully, the capabilities of Aron’s plugin will become part of the core Thunderbird product, soon. But, since its been almost 7 years since this bug was reported, completely hassle free interchange of messages between Thunderbird and Outlook users may still be a way off.