Earthlink Mail - Can't figure out what this file is

All,

I've got a case where the guy is using Earthlink for Internet access, and Earthlink Mail for his e-mail client. He only has 4 e-mails in his Inbox, but it appears that he has a "history" of past e-mail activity. There is a file called

"\Documents and Settings\account name\Earthlink\6.0\Identities\account\mailbox\Index\ElnkMail-words-bulk-0.dat"

I have no idea what this .dat file is, but it contains a web forum screen name that is of interest (it's the screen name he registered). There's no other trace of his screen name on his computer, so I'd really like to be able to explain what this file is and how the user name got there…

The .dat file wasn't erased or overwritten…just fyi.