Disappearing Outlook Express Read Receipt

I sent a message requesting a read receipt (it's automatically set up that way). The message was read and the read receipt was sent back simulataneously as usual to my blackberry and my computer Outlook Express (I think it's version 8). It was for a meeting acceptance. I saw the read receipt myself on the blackberry, and then about 15 minutes later the read receipt was gone--from both the blackberry and the computer. What happened to it? All the other read receipts I've gotten from this person for the last year are still there.

So... where did this one go?

Thanks