Free/Busy view from outlook to Notes not working for all users

I needed some times to realize that the free/busy view from Outlook wasn't always working when including Notes users.

I figured out the source of the problem: The System Attendant Must have a Lotus Notes proxy address.

To check if the System Attendant got a Lotus proxy address, fire up adsiedit.msc,
then go to:
Configuration / Services /Microsoft Exchange / Your Org name / Administrative groups / Your administrative group / Servers / Your Server with lotus conn / Microsoft System Attendant

On the properties page, go to proxyaddress, then edit and you should have something like:
NOTES:Microsoft System Attendant/First Administrative Group/@Exchange

I didn't have, so i added it manually and it worked out !

Microsoft KB about this issue: Click here