If you’re using an EAS 16 backend, and a SEG with attachment or hyperlink transformation delivering mail to the native iOS mail client, you will see the mail app crash on iOS 9. AirWatch are working on a fix, for more details see https://support.air-watch.com/articles/96223778-Attachment-hyperlink-transformation-failure-on-iOS-9-when-using-SEG-and-Office-365.
To workaround the issue, either turn off the attachment or hyperlink transforms, tell clients not to upgrade to iOS 9, or use a device like an F5 APM to re-write the ASProtocol header coming from your backend to EAS 14.x.
The issue comes up in iOS 9 because the mail client now supports EAS 16, in iOS 8 the maximum supported was EAS 14.