The company I work for got caught with this.
Email with subject "Monthly Statement" from a known supplier was opened by someone in A/P, so it's hard to blame them.
Locked about 4TB of Data but was mainly word/excel/eps files that wouldn't be updated that frequently. Didn't effect SQL or mailboxes. Restored from previous nights backup. Absolute pain in the arse and took me a good few days to get everything back. Most users didn't really notice a problem.