Spam Filtering on exchanger server

We have 300 users in organistion and all users get connect to Microsoft outlook to send/recive emails however, now a days I got to hear alots of issues related to spam mails. Users receive alots of spam mails into they mailbox and some of the unknown user have subscribed into unappropriate yahoo groups and given certain distrubtion group which users get to receive many pictures/junk mails into user mailbox I wasnt sure users which email address they have been subscribed to receive mails which I need to avoid in furthur and have users safe mails. Is there any solution which I should be able to implement on exchange server 2003 to avoid these mails.

Your help is greatly appreceiated