According to the Rackspace Email & Apps System Status page:
We will be conducting a scheduled maintenance 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 March 27th, 19:30 CST (00.30 UTC) to 21:30 CST (02.30 UTC).
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!