I have the same problem and it definately has to do with a configuration profile via MDM or Apple Configurator.
What I found is that if you setup an email account with a configuration profile the MFMailComposeViewController returns false from canSendMail if this email account is the only one on the device.
The option to restrict the email to only the mail app, had no effect on that. It was present wether the option was checked or not.
I filed a bug with a sample project that illustrates that. The report id is 30443554