One of problems found at the use of iOS 9 is email messages that cannot open. This problem makes some people confused because they cannot find their email messages although those messages are already downloaded.
This problem usually happened on the messages with an attachment, and a notification usually popping up says: “This message has not been downloaded from the server“. Some people find that they can access their downloaded email for 24 hours then they can’t access to it after that first 24 hours.
This problem is related to POP email accounts that does not specified accordingly and causing the downloaded messages deleted automatically from the server.
To fix the problem, you can try to change your all devices setting in accessing your POP account so that it not deleting your messages from the server after your device receiving those messages.
Please go to Settings > Mail, Contacts, Calendars, and choose the email account in question then tap Advanced > Delete from server and please choose Never on it. Those steps are quite successful to be practiced at any other devices that access your account although it may disturb the purpose of your POP account.
You also can save the attachment of your message in other folder of your phone other than in your email message so that you have the backup data of your email before it deleted by the server.
Many people try to get the potential solution to fix problem by following these steps: deleting their accounts then back to set them up again from scratch, resetting devices, and restoring the backups to get their messages back.
However, most of them reported that those steps are not working and if the messages are deleted from the server, those messages cannot be retrieved again. You can report this problem to Apple directly via web or report it in an Apple Store.