As a side note, the OUTLOOK.NK2 file right next to it is your cached email list. I find users are happy when I copy that over as well. It contains the entries in the drop down list that shows up when you start typing in email addy's and shows the email addy's that you have used before.

If you are moving files to a new machine you can just copy the old NK2 file right over the top of the one on the new machine.

Here's a better explanation directly from the source:
http://office.microsoft.com/en-us/outlook/HA011394511033.aspx