Yes, we know Thunderbird and evolution are excellent email client software can replace part of Microsoft outlook, but there disadvantages, the winmail.dat attachment is a serious issue.
some attachment display in Thunderbird as a file winmail.dat why all attachment can be displayed in Microsoft outlook correctly, these winmail.dat are generated by Microsoft outlook.
we can install some 3rd part software such as winmail_opener.exe or WMdecode.exe, but it makes end users confused. we need a email client that can display email attachment correctly that sent from Microsoft Outlook 2003.

i had tested following email client with winmail.dat
1. Thunderbird. can't open
2. evolution. can't open
3. foxmail unable to test because it does not support IMAP
4. windows live mail unable to test: it display error on first run, code (0x80048883)

there are some solutions from moillazine.org http://kb.mozillazine.org/Winmail.dat_attachments

install another software or disable TNEF in outlook, i think the second solution is a joke.

so there is only one solution, install one of the following program,
1. LookOut extension
2. Winmail.dat Reader
3. Fentu
4. AkeeSoft WMViewer
5. Winmail Opener
6. TNEF linux
7. TNEF's Enough for Mac OS

the first one is my choice. the extention can be downloaded from Https://addons.mozilla.org/en-US/thunderbird/addon/4433