MFMailComposeViewController canSendMail returns false even when email is configured

Does anyone have ideas of what would make MFMailComposeViewController canSendMail be false, even when there is a working email account configured? My app has a feature to allow the user to report problems via email. I check MFMailComposeViewController canSendMail and show or hide that button depending on the return value. I know this user has email - he has sent me email from the device mail app, but he says my 'report problems' button is not showing.