Exchange 2007 email logging

I'm trying to activate the email logging functionality using NAS, but I always get an error message in the Job Queue Log Entries:

This message is for C/AL programmers:

The call to member Logon failed. Collaboration Data Objects returned the following message:
[Collaboration Data Objects - [E_FAIL(80004005)]]

The setup involves 3 machines:
- the Exchange 2007 server
- the NAS server
- my laptop running Outlook and the NAV 2009 client.

The user set in the NAS service has domain Administrator rights, is a Super user in NAV, has owner permissions in the email logging Public folders (Queue and Storage) in Exchange.

Any idea?

Thanks a lot

Michele