Ok, this is somewhat awkward.
On Tuesday, Apple issued a system status update on the iCloud Support web page that confirmed that some users may be affected by a temporary error where older emails are inaccessible.
The message, posted under “iCloud: Mail & Notes” shortly after noon PST on Tuesday, said select iCloud users may not be able to access older email messages, though sending and receiving new messages should be unaffected.
Apple has promised the issue would be resolved “ASAP” but gave no indication of what exactly caused the error.
Stay tuned for additional details as they become available.