Hi Alain,
It seems the problem is related to the Outlook 2011 client. You can create a new identity to troubleshoot this problem. The following article describes the detailed steps and the possible problems caused by the identity database damage:
http://support.microsoft.com/kb/2360509
If the issue persists on multiple client, you may consider asking this question on Office for Mac 2011 forum.
Here is the link of Office for Mac forum for your reference: http://answers.microsoft.com/en-us/mac/forum/macoffice2011
I have also found there is a similar thread, you can refer to it by the following link:
http://community.office365.com/themes/groups/generic/forums/thread.aspx?ForumID=158&ForumThreadID=155976&GroupKeys=en-us/&PageIndex=1
Thanks,
Jason Jiang