Read mail turns BOLD again

I recently reconfigured Outlook 2000 with some users. I deleted the local folders so the users have their mail on the server only. Now this weird thing happens, when a user read its mail and press send or receive to transfer new mail the old mail start to double itself until all mail is BOLD again(un read) and deletes the mail that was read. Like he did not read its mail before.
Later when moving the read mail to another folder, this problem stopped.
Exchange is version 5.5 with sp3
Why this problem?