Reply #134: It's a number to account for all the messages found.

[boppers]

One email message, about, oh, a staff birthday party gets sent. Say there's 200 recipients. One message is suddenly 201 (one on the server, 200 on the recipient computers). Somebody hits "reply to all", and says 'Hey, I'll be there!" Now, there are two actual messages, but counting all messages on all backups, that's 402 messages (the envelopes will be different).

They aren't just harvesting messages from the servers, they're pulling all the workstations, too (in case somebody used an external SMTP/IMAP/POP (etc.) mail service.