I found this in a newsgroup:
Numerical Code: 5.1.6
First Available: Exchange 2000 Service Pack 2
Possible Cause: One possible cause is that the user directory attributes such as homeMDB or msExchHomeServerName might be missing or corrupted.
Troubleshooting: Check the user directory attributes integrity and rerun the Recipient Update Service to make sure attributes required for transport are valid.
Use ADSIEdit and look at the attributes of the specific public folder in question and see if these attributes are correct.
Hope this helps…