Need to use an MTA with Fetchmail??

I have fetchmail setup to download messages from my companies ISP. It is doing that correctly for my test account. However when I use the email client (Outlook in this Case) i cannot download the messages to Outlook. Am I missing a part here. Do I need to also be using Sendmail or Postfix along with fetchmail in order to allow Outlook to download the messages. Although I am not new to Linux this is my first attempt at something like this so any additional help or ideas would be very helpful. The end result I want is to use SpamAssasin to filter Spam before the User can download it. Thanks as always, Aaron