My suggestion is to add a dialog when the user clicks send (validating the mail) that warns about an empty To: field that this may cause issues for some MTA and/or email clients. This dialog could also suggest adding the sender as the recipient of the To: field. The dialog could then have three buttons: Cancel, Send with sender as recipient, Send. Default being Send with sender as recipient.
My suggestion is to add a dialog when the user clicks send (validating the mail) that warns about an empty To: field that this may cause issues for some MTA and/or email clients. This dialog could also suggest adding the sender as the recipient of the To: field. The dialog could then have three buttons: Cancel, Send with sender as recipient, Send. Default being Send with sender as recipient.