Getting default credentials, like Outlook

I'm developping, in c#, an application that will (among other things) send e-mail.

The idea would be to have -- for example -- an "order this item" button, and when they click it, 开发者_运维技巧the system sends an e-mail to some Exchange e-mail group with the item's ID.

The deal is the following:

• users have logged into Windows with some credentials. I'd like to avoid people needing to re-type these credentials in my application
• I'd like that the person who receives the e-mail see the currently logged in user as the author of the e-mail
• We use Outlook 2003 and Exchange Server 2003.

The thing is, when we set-up outlook, we only need to type in the server address and the full name, but the system never asks for a password...

Just wondering, why do you need to use a client side program like Outlook to send the email, why not just use System.Net.Mail provided by the framework.