A family of Microsoft word processing software products for creating web, email, and print documents.
It should not drop out of the file, because it's an integral part of it. Word places it on the header as a WordArt object.
Is it possible that your adressees cannot see it because of their Word settings? (Namely Preferences > View > Drawings on the Mac, Tools > Options > View > Drawings on XP, etc.).
Viewing the document in Draft view (old Normal view) won't show the watermark either.
Email the file to yourself, open it and check whether the watermark has disappeared. I don't think it will.