search functionality doesn't work correctly when using another mailbox as primary

Hi there,

One of our customers has a Exchange 2010 mail system.
All of the employees use Outlook 2010 to access their mailboxes.

We have one shared mailbox that is added as a extra mailbox for a lot of users from this domain, this mailbox is called "info".
2 of these users want the ability to primary send all their outgoing e-mail as this "info" mailbox.

The solution for this is to use the "info" mailbox as their primary mailbox, this is working fine but there is one problem:
the search functionality doesn't work correctly.
When you use the search ability from outlook I found out that the search results doesn't get displayed as they should, for example we search in the contacts:

when you search for"the term "forklif" it doesn't display any results
when you search for the term "forklift" it show about 20 contacts

I've found out that when these 2 persons use their own mailbox as primary and add the "info" mailbox as an extra mailbox the search fucntionality works correctly.

does anyone knows what is causing this problem?
I've already turned off indexing and caching features but this doesnt make any difference.

some extra info:

The "contacts" folder from the "info" mailbox contains over 8000 items (8k).