OS X Calendar email alerts — send via Outlook?
Either you dummy configure Mail so you can access Preferences from the Mail menu and set in the General" pane your default email reader (if not working from Outlook preferences).
Or you may type in the following in Terminal:

defaults write com.apple.LaunchServices LSHandlers -array-add '{ LSHandlerURLScheme = "mailto"; LSHandlerRoleAll = "com.microsoft.Outlook"; }'


This should add an entry to the "~/Library/Preferences/com.apple.LaunchServices.plist"

Using Xcode you add a dictionary item with the two following items:

&lt;key&gt;LSHandlerURLScheme&lt;/key&gt;
&lt;string&gt;mailto&lt;/string&gt;




&lt;key&gt;LSHandlerRoleAll&lt;/key&gt;
&lt;string&gt;com.microsoft.Outlook&lt;/string&gt;

Using RCDefaultApp could do the trick but since it hasn't been update since 2009 I'm not sure it's still working.