According to the Rackspace Email & Apps System Status page:
Scheduled Maintenance: Authentication Service We will be conducting a scheduled maintenance on the authentication services for our Hosted Email environment. During this time users may experience intermittent failed authentication attempts when accessing their email over IMAP, POP, SMTP, or Webmail. Incoming email delivery and Hosted Exchange Outlook/OWA clients are not affected.
This change will happen Wednesday January 16th, 20:00 to 00:00 EST
Please continue to check my blog for service updates. As soon as I become aware of an issue that may impact your website or email, I will post it here. Thank you!