Where does Outlook Express keep a mark of emails downloaded using POP3 protocol?

"This file ""Pop3uidl.dbx"" file in your Identity's store folder keeps track of which messages on the server have already been downloaded. If you delete ""Pop3uidl.dbx"", the next time you check for mail, all the messages on the server will be downloaded again, and a new ""Pop3uidl.dbx"" will be created.

Should you wish to analyze the contents of this file, Arne Schloh has published his analysis of the dbx format here. But you may be able to short-circuit the search for these bytes, by copying Pop3uidl.dbx aside, downloading some messages, then doing a binary compare between the before and after copies of Pop3uidl.dbx. This might give you the byte offset of the information that you seek."