AS penance for incorrectly calling the resolution of the issue following the iOS 9.1 upgrade I have spent the evening drafting a new fault description for the apple feedback site. Feel free to copy from this post and submit as many times as you like at http://www.apple.com/feedback/
Subject:
iOS 9 issue with pop/smtp email
Description:
Following upgrade to iOS 9, the content of emails downloaded to pop/smtp email accounts are randomly lost after roughly 24 hrs. The email content is replaced by the message: This message has not been downloaded from the server. The same email accounts on an iMac running OS X Yosemite & another iOS devices still running iOS 8 do not exhibit this behaviour. The 4 iOS devices upgraded to iOS 9 (iphone 6 & 5c, iPad 3 & air2) all exhibit the same behaviour.The same behaviour has been confirmed at iOS 9.0, 9.01, 9.02 and 9.1. Unlike ios8, when mobile data is not available on the iOS9 devices image content is lost from downloaded emails.Loss of the email content is unrelated to emails presence on the email provider's server. This iOS9 bug has resulted in the loss of irreplaceable user email data.