When you right click on text in Outlook (or in Word) one of the options in the context menu is "Search with Bing". If you prefer to use a different search engine, you can do so by adding two values to the registry, one for the display name and one for the URI.
You can use any search engine you prefer. If you aren't sure what URL to use for your favorite search engine, look at the URL in the address bar after doing a search. Strip away extra junk in the URL, leaving the base URL and the query string (often ?q=), as the total length of the URL cannot exceed 253 characters. The change takes effect immediately, you do not need to restart Outlook.
Open the registry editor and browse to Add two string values to hold the friendly name and URI.
Value: Friendly name
Value: Search Url to use
The urls for these popular search engines are below
|Search Engine||URL to use|
If you prefer not to edit the registry yourself, I have ready-to-use reg files for these 3 search engines at Change the Search Provider in Office.