Hi,
If the drop down has the email address as the value of the items in the drop down (not the text of the items) then you can use code like;
var vEmailTarget = "mailto:" + this.boundItem(xfa.event.change) + "?subject=email subject";
EmailSubmitButton1.event__click.submit.target = vEmailTarget;
In the change event of the drop down. If the email address is what is displayed in the drop down then change this.boundItem(xfa.event.change) to just xfa.event.change.
Regards
Bruce