A good place to start would be on their PC and their email client!
How are they accessing their email? Web Mail or Windows/Mac email client (Outlook, OExpress)?
Are they always accessing it from the same machine?
I would be tempted to look there before examining your code if you are not seeing the same results.
How did it test when you were trialling it? Have you done any significant alterations to the css of the template?
Check out the template email file: 'WA_Universal_Email/Templates/contactus,php' and see how that looks.