how to have drop down list update automatically

I want my drop-down list (that shows up when I am sending a new email and
click on "To") to automatically update when I change a distribution list in
my contacts. I work in a business where people come and go all the time, so
I update the distribution lists, then share the updated ones (as attachments)
with my coworkers who delete their old ones, and drag the new one into their
contacts. Often, they end up using an old list because they choose a list
from the drop-down which is an old one. Anyone have experience with this?