Followup to "I can't see the headers from one sender in Leopard Mail"

[PeterB]

Doc wrote:
Quit Mail.

If you haven't deleted the entry and re-entered the contact info for your friend in the Address Book, do so now and then quit the Address Book.

Go to "~/Library/Application Support/AddressBook"

Remove these 2 files:
MailRecents-v4.abcdmr
AddressBook-v22.abcddb

Launch the Address Book to rebuild its index.

Launch Mail.

Can you access Previous Recipients now?

Bingo! That did it, thanks so much.

BTW, the person in question has never been in my Address Book.

Now, removing her from the Previous Recipients list, I can now see all her emails and their headers.

So... what to make of this? What exactly was the problem, a corrupted pref file as I suggested?