Outlook may be configured to convert inbound mail to text. If that's the case, there is nothing you can do to fix the problem.
Check your Domino server 'configuration - Mime - Conversion - outbound' settings and make sure you're pushing out HTML. We do this and even doc links survive the trip to Outlook.
If you want to eliminate the hassle, go to Genii software; Ben has a wicked awesome product that maintains email fidelity across platforms. (don't work for the company, just know Ben makes wicked awesome products that solve real problems).