this is what I did recently in a before insert business rule on sys_email -
var usermails = current.recipients.split(",");
var usermails1 = current.recipients.split(",");
for(var i=0;i<usermails.length;i++){
var gr = new GlideRecord('sys_user');
gr.addQuery('email',usermails[i]);
gr.query();
if(gr.next())
{
var cuser = gs.getUser();
cuser = cuser.getUserByID(gr.user_name);
if (your conditon to remove the recipient)
{
usermails1.splice(i, 1);
}
}
}
current.recipients = usermails1.toString();
