//Approach 1, simply mention from and to as external email addresses
var docMail : NotesDocument = database.createDocument();
docMail.replaceItemValue("Form","memo";
docMail.replaceItemValue("From","abc@gmail.com";//assume this is actual mail id
docMail.replaceItemValue("To","def@gmail.com";//assume this is actual mail id
docMail.replaceItemValue("Subject","Test mail";//assume this is actual mail id
docMail.send();



//Approach 2, suppose I am logged in as test user1/Dev
//Configured forwarding address for test user1 and test user2 as     abc@gmail.com and def@gmail.com respectively.

var docMail : NotesDocument = database.createDocument();
docMail.replaceItemValue("Form","memo";
docMail.replaceItemValue("To","CN=Test User2/O=Dev";//assume this is actual mail id
docMail.replaceItemValue("Subject","Test mail";//assume this is actual mail id
docMail.send();