Apple confirms select iCloud users unable to access old emails
The message, posted under "iCloud: Mail & Notes" shortly after noon PST on Tuesday, notes select iCloud users may not be able to access older email messages at this time, though sending and receiving new messages should be unaffected.
Users reporting problems with Apple's cloud service have complained that messages received prior to July 30 become unavailable and disappeared from their devices after syncing with iCloud. It is not yet known whether the messages have been deleted or are merely not being pushed to users' devices.
Source: Apple
Apple has promised the issue would be resolved "ASAP" but gave no indication of what exactly caused the error.