The company says the outage had no impact on Rackspace Email or any of its other products.
An emailed statement said: “On Friday, December 2nd, Rackspace detected suspicious activity on its Hosted Exchange environment.
“Upon discovery, Rackspace immediately took proactive measures to isolate the Hosted Exchange environment to contain the incident, working alongside industry-leading third-party cybersecurity experts.
“The ongoing investigation has determined the activity to be the result of ransomware. Our technical teams are working diligently to help affected customers migrate to a new environment as quickly as possible.
“Based on the investigation to date, we believe that this incident was isolated to the Hosted Exchange business.
The attack may result in a loss of revenue for its Exchange business which brings in around US$30 million annually.
Rackspace says it has hired a leading cyber defence team to assist with its investigation into the ransomware attack.
The investigation is still in its early stages, however, but the firm has pledged to notify customers as it continues to look into the matter.
