Quick demonstration -
1) Create the url request:
var u:URLRequest = new URLRequest("mailto:"+EMAIL_ADDRESS);
2) Create a variable for the subject and body:
var v:URLVariables = new URLVariables(); //interestingly, you can put all the variables into a string and pass that in, but here we're just going to append them as properties to the object
3) Assign those properties:
v.subject = "Application crashed";
v.body = TEXT_OF_CRASH_MESSAGE;
4) Pass those variable properties into the URLRequest:
u.data = v;
5) Navigate to the URL:
navigateToURL(u, "_self"); //the _self is .js and not required
This is an easy way to pass any valuable info from your flex app into your user's default email program (Entourage/Outlook, Thunderbird, Mail, etc.)