These constants define if wodSmtp will insert specific
headers into the message that define if receipt is
requested from the server/recepient when message is
read/delivered.
OnDeliverySuccess, OnDeliveryFailure and OnDeliveryDelay will cause wodSmtp to send
NOTIFY=SUCCESS,DELAY,FAILURE immediately after specifying
RCPT TO command.
OnReadMessage will cause wodSmtp
to insert 'Disposition-Notification-To' and
'Read-Receipt-To' headers into the message.