Cisco Firewall :: ASA 5510 - Setting Up SMTP Port Block?

how to go about setting up the ASA to block any SMTP traffic outbound except for our Exchange Server. This is in relationship to a SpamBot issue that blacklisted us. I have an ASA 5510 running version 6.2(5) / 8.2(2) with three ports. DMZ, Inside and the Outside interface. Up till today, I only needed to block outside traffic to our internal network which I used the ASDM to configure a rule on the outside interface for an incoming rule. I am assuming I need to create an outgoing rule on the outside interface; however, just to make sure I understand the terminology/traffic flow, I created the rule with my computer as the source (192.168.0.131) with ALL destination and the service as HTTP. My logic, which seems to fail here, is that any traffic from my computer going outbound would be blocked; however I am still able to browse... That said, if I were to change the source as the Exchange server and the Service Type to SMTP, it would not actually block traffic and therefore not solve our problem.  I even gone as far as permitting traffic from my computer, expanding the hit counter and I see no hits.  So I am no doubt doing this wrong. What I do know, is when I first created the rule, a second rule was automatically created (Implicit rule) that deny all sources and blocked all HTTP traffic until I changed it to Permit?