If you are using Rackspace hosted email, you may experience issues trying to log in to webmail.
This only appears to be affecting webmail login. POP/IMAP (eg, via Outlook) is unaffected.
The following notice has been posted on their status page.
Emergency Maintenance - Hosted Email
We will be performing an emergency maintenance to enhance the systems responsible for managing webmail login requests. The maintenance window will begin Friday August 24, 2018 at 12:00PM and end on Sunday August 26, 2018 at 6:00AM.
While all measures are being taken to prevent any impact to users, some may experience a brief disruption of service during the maintenance window.
We encourage administrators with users experiencing issues once the maintenance window concludes to contact their Support Team for further troubleshooting..