According to the Rackspace Email & Apps System Status page:
We will be conducting a scheduled maintenance on our Hosted Email environment. During this time users may experience connectivity issues 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 Thursday May 2nd from 19:30 CST to 22:00 CST.
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!