Update: Apple says that the issue has been resolved.
Users may be unable to send or receive mail and may also be unable to use Notes.
According to Apple, only 0.86% of iCloud users are affected by the outage, which has been ongoing since just after 8 a.m. EDT. The remaining iCloud services, including Reminders, Contacts, Calendar, and Drive, all appear to be operational at this time.
We'll let you know when Apple offers an update on the outage.