Apple has confirmed an issue recently reported on the iCloud support forums, which is preventing users from accessing any message received prior to yesterday — Monday, July 30th. Multiple users claimed that messages synced with iCloud disappeared from their devices, and Apple has promised to return these messages saying the following on its iCloud Support page:
A small number of users may currently be unable to access older email messages. Sending and receiving new messages is unaffected. Access to older messages affected by this issue will be restored ASAP.
While users await the return of their older messages, some claim to have found interim solutions, such as restoring their Mail client from a Time Machine backup. However, other users are claiming that iCloud will immediately delete the emails once a sync has occurred. Though no service is infallible — Gmail has encountered similar issues in the past — the age-old principle of keeping a local backup still persists even in the age of cloud computing.