I just upgraded to Parallels 11 last week, and have been having trouble with my windows switching when mail arrives in outlook. I am running on an OS X host, and have two Windows 7 guests. One of them has Outlook 2010 running, and whenever new mail arrives, the screen switches over to that guest. I have disabled notifications, both in Windows, and in OS X, for Outlook, but it still switches over. It only happens when the active screen is the other Windows guest machine (the one without Outlook), not when using an OS X application. Since I get a lot of email, this is unusable, since I have to do a lot of work in the second guest, and every ten or twenty seconds, it switches over, so I need to close Outlook in order to get work done. This did not happen in Parallels 10, or any previous versions.
Follow the below step as a workaround in the Virtual Machine in which Outlook is installed. 1. Click on Actions > configure > options > web and email > email and change it to open in Mac.
I figured it out. I had to disable Outlook notifications in notification centre on my mac. I'm not sure why this just started happening after the upgrade, but it was a huge annoyance.