Apple has updated its System Status page to reflect that iCloud Mail and Notes may currently be unavailable for approximately 0.86% of users.
Apple says "users may be unable to send or receive mail and may also be unable to use Notes" until the services are restored.
The outage began shortly after 5:00 a.m. Pacific. This article will be updated when new information becomes available.
Update 9:41 AM: According to Apple's System Status page, the issue was resolved shortly after 9:00 AM Pacific Time.