This post is common knowledge to all SharePoint professionals out there, but for newbie's it can save hours of hair pulling (as I’ve got to learn from a bunch of people I’ve helped out) ;)
Scenario: There is a distribution list you can see and use in outlook, but when you search for the same list in SharePoint it doesn’t show up in the people picker. What gives?
Chances are that the distribution list is not security enabled. This would need to be done by your domain administrator. He would need to change the ‘Group Type’ of the list from ‘distribution’ to ‘security’. Once this is done your SharePoint administrator would need to run a profile import so SharePoint can pick up the change immediately (alternatively you can let SharePoint automatically pick it up during its scheduled AD crawl).