The email servers were hacked and luckily, it seems that only 6 clients were impacted. Deloitte has since implemented multi-factor authentication and contacted law firm (Hogan Lovells) for advice and conducted a forensic investigation.
They didn’t know when the hackers got in and where they went and the trace left is complete. It remains me the intrusion detection classes I took that mentioned a similar case and the importance of backup.
https://www.theguardian.com/business/2017/oct/10/deloitte-hack-hit-server-containing-emails-from-across-us-government