When someone sends me a video clip attached to an email, RealPlayer won't play it. Why not?
Generally speaking, RealPlayer should be able to play video clips sent via email, as long as the file type is supported. You may need to download and save the file to your computer first, before trying to play it in RealPlayer. Read more about supported file types.
Also, it's possible that even a video saved as a supported file type is using a codec (compression/decompression system) that RealPlayer doesn't recognize. This answer may help to resolve this issue.