According to the Rackspace Email & Apps System Status† page:
We will be conducting scheduled maintenance on the authentication services in our hosted email environment on Wednesday May 15th 19.00-21.00 CST. 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 will not be impacted.
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!