Which version of BES do you have?
I had a support call open to RIM on this very same issue, they claim it's a defect in 4.1 versions prior to SP7. I found that this usually happens when a device roams on wireless network that doesn't support BES data such a cruise ships and foreign countries but at times I see this too just when a device is down and powered off for a a period of time.
For my site, I had both 4.1 and 5.0 BES for GW running. How I found it and fixed it (for the user is question) was to move the user over to the 5.0 GW BES using the Transporter and then edit the default config for that user as I discovered that even after moving a effected account over to my BES 5, they still didn't have inbox re-direction but you can re-enable in the BES 5 web interface were you CAN'T in the 4.1 no matter what you try.
RIM states you need to apply the latest SP7 and other hot fixes, I'm in the middle of migrating to BES 5 so I'm just moving my users over off the 4.1 server.