An optional feature of email is a header that requests a return receipt once the recipient opened the email. You can add this header by using the otherhead$ template variable or email command/function parameter.
Call "mailcall.bb",1,t$,e$
t.to$="some-email-address"
t.subject="test message"
t.otherhead$="Disposition-Notification-To: " + "return-to-email-address"
call "mailcall.bb",0,t$,e$
Note that return receipts are requests, and the recipient is not required to honor the request.