Although Outlook can be set to access a certain email account depending on the internet connection, the default setting is to access whatever email you set up regardless of the internet connection. In short, tethering with the BB does not have anything to do with Outlook unless you set it up that way.
The reason why craigslist is forcing you to use Outlook is because when you click on a reply link the actual link is formatted to open your default email client. When you hover your mouse over the reply link, in the lower left corner of the browser you'll see mailto:whoever@domain.com which normally opens Outlook. Since you don't use Outlook for email and I would suspect you log into a web site like yahoo, hotmail, comcast, etc. Instead of clicking on the email address, right click on the reply and copy the link. Then go to your regular email site, create a new email message and right click/paste the link in the TO field, deleting the mailto: portion. Or just type out the email address that you see.
If you actually want to use Outlook for email, you need to go to you ISP's site for the POP settings. There are advantages using an email client, but that's for you to decide. I use it for a few reasons, one being I hate logging in to a slow/buggy website every time.
How to set up Outlook Express
Hope this helps.
Setting up Outlook express
