I'm having the same problem at my office. We all use Macs here, but our clients use PCs. Anytime we send out an email that has been typed out in MS Word v. X, it has question marks randomly throughout it, no matter whether the client is using Outlook or Outlook Express.
If the email is created in the Mail application or TextEdit, we don't seem to be getting the question marks.
This has been a recent development though. All through 10.3 and most of 10.4, this has not been a problem.
Does anyone know why this is happening?