According to Apple’s iCloud status website, there will be a brief two hours of maintenance tonight. This may result in a small percentage of users seeing e-mail downtime:

iCloud: Mail & Notes – Maintenance – 07/10/2012 22:00 PDT – 07/11/2012 00:00 PDT – Users Affected: 0.5%

07/10/2012 15:55 PDT

Users may be unable to access iCloud mail. Normal service is expected to be restored at the end of the maintenance window.