Apple introduced ‘Data protection’ feature in iOS 4.0 devices that offer hardware encryption for all the data stored on the devices. But unexpectedly, In last few updates Apple has silently removed the email attachment encryption from data protection mechanisms. That means, email attachments are not encrypted on the iOS devices by a unique 256-bit crypto engine, and if. someone else gets access to your device, he will be able to get access to. your private content, he explained in a blog post.
Source: https://thehackernews.com/2014/05/ios-apple-iphone-data-protection-email-attachment.html