I agree with Frank about HTML mails. They are a PIA.
About the actual problem, Dhanasree will have to choose between a text based one or an HTML based one.
Meanwhile, I am assuming that you are talking about web-version of Outlook client. So one solution is send HTML always, and if users Outlook client is configured to NOT to accept and display HTML mails, let them open it in a new browser window, and then onwards all the links in the mail will open in that window.
Or, go simple text way, where you will have the actual mail displayed in Outlook regardless of the client configuration, and every link will open in different windows.
