You can't with just FileMaker. However, there are several Email plug ins offered by third party developers that offer this capability. Do a web search for FileMaker Plug ins and check them out.
Thanks, I found something called entourage manipulator, and seems to be what i am looking for, since I use entourage as my default mail applicattion. Although setting it up seems for someone more advanced in file maker scripting. have you seen this plug in?