Its is a confirmation email, that you printout. The barcode is already displayed (saw that on another's email).
Example, a confirmation email, print it, and then when you attend meeting you scan barcode for an admission badge,
It sounds like it's just not fully downloading your email. You can force it to download it by pretending to forward the email. The only setting I can think of that could affect it would be "load remote images" under the "Mail, Contacts, Calendars" settings.