The DSNReturn property specifies whether or not the message should be included in any failed DSN issued for this message transmission.
Sets or returns a String value.
The DSNReturn property may be set with on of the following keyword:
FULL requests that the entire message be returned in any "failed"
delivery status notification issued for this recipient.
HDRS requests that only the headers of the message be returned.
The FULL and HDRS keywords may be spelled in any combination of upper
and lower case letters.
DSN stands for "Delivery Status Notifications", it is an ESMTP
extension that is implemented by most modern SMTP servers. See RFC1891
for details.