Turning on activity indicator for in app email

I'm sending an email from within my application. There is an attachment and I wanted to show the user that the email is being sent because it takes a few seconds after I dismiss the MFMailComposeViewController.

Does anyone know how I can turn on (and off) the status bar activity indicator based on when the email starts and finishes sending? I thought the MFMailComposeViewControllerDelegate would be a good place to look, but I couldn't find anything there.