Thank you very much for the answer, I found a good way to solve the problem. Others have recommended an APP to me. After I used it, I can solve the problems of winmail.dat files. If anybody need help, I can tell you this app is Winmail DAT File Viewer: Open DAT,XPS,MSG files. You can find it in Mac App Store,
https://itunes.apple.com/us/app/winm...30699881?mt=12.
I hope it can help someone who like me about winmail.dat files.