According to user reports, iCloud mail went down around 9:30 AM Eastern Time this morning, and it remains out some seven hours later. Apple updated its iCloud System Status page at 10:08 AM Eastern Time to note that Mail and Notes were down for some users, but pegged the issue as affecting less than 1% of users.
iCloud services do go down from time to time, although the downtime is usually fairly brief, as evidenced by a short half-hour outage just yesterday. And while Apple's status update indicates that only a small number of users are being affected by this latest outage, for those who are unable to access their mail the extended seven-hour downtime may be a significant inconvenience.
Apple has provided no estimate of when affected users can expect iCloud mail services to return, noting only that they "will be restored ASAP".