When I type in the email address of a contact and also of a user in the global search, it does not return any records for either contact or user although there are both a contact and user record with this email address. Is this a feature limitation in Salesforce or do I need to make config changes to enable this level of searching?
Make sure your search is not being filtered by anything else. You can see that by clicking on Options, if you are using divisions, you need to be searching on all divisions or on the correct division. Notice the "limit to items I own" and "Exact phrase" filters.