I was hoping you found a solution by now. This has been happening to me
since April.
I found a post dated 4/19/2005 entitled, "Sent e-mails are being deleted
after 7 days." It says anti-virus programs that have e-mail scanning can
cause problems, especially in Outlook Express. Disabling the feature doesn't
always work. Sometimes you have to reinstall the program without installing
the e-mail scanner.
I wonder if this is our problem?