When you receive a new email message in Microsoft Outlook for Mac 2011, the main Outlook window takes focus from any other applications in the foreground. This causes the Outlook window to jump in front of any other active applications.
This issue occurs for one of the following reasons:
A corrupted Office for Mac 2011 installation
Corrupted preference files
To resolve this issue, uninstall Office for Mac 2011 and then reinstall it. To do this, see the following article in the Microsoft Knowledge Base:
2398768 How to completely remove Office for Mac 2011
Article ID: 2744147 - Last Review: 29 Aug 2012 - Revision: 1