This doesnt make sense to me. Attaching it to email does nothing to change the file structure or its contents. Can you verify this or is this happening on a clients or someone else's machine?
Make sure they are not opening it in Novell or Outlook and are downloading it to their machine first. The state system here is set up with Novell and it gives you the option to open the attachement within Novell utilizing the host program and it is horrible. I don't know why Novell ever thought this would be a good idea, and on that note, I don't know why the state ever thought using Novell would be a good idea.
Edit: Also, make sure that the machine that recieved the attachment can handle the wmv file at the bitrate it was encoded. If it is an older machine and a lot of resources are tied up, it could cause a problem with playback- but probably more so with stuttering.