Another bug with iOS 9 has been found that some iPhone users have issues with POP email accounts and attachments in iOS 9 after it was released to the public. Users with POP email accounts have been having trouble opening emails with attachments.
Customers with a wide range of devices are receiving a "Message not yet downloaded from server" error whenever they attempt to open an email message with an attachment. The problem appears to be affecting Apple customers who have POP email accounts and is causing issues with both new and old emails, according to MacRumors
Deleting email accounts and adding them back in does not fix the issue, but downloading an alternate email program from the App Store seems to have worked for many users as a temporary fix. Turning off Wi-Fi also allows some users to read their previously downloaded emails and access attachments, but images are not viewable.
One customer experiencing issues received a response from Apple's engineering team, suggesting the email issue occurs when an email account has been configured to remove a copy of emails on the server. This has left some wondering whether the behavior is an intended function or a bug, but while leaving messages on the server solves some of the problem, sent and trash emails remain unaccessible. The requirement to leave messages on the server also defeats the purpose of a POP email account.
Anyway, Apple will give a new iOS 9 version to fix this problem in the future. So wait and don't panic.