Delivery Status Notification (DSN) replaces certain SMTP error codes and the Return-Receipt-To: header (Return-Receipt-To:) as a means of handling multiple delivery status requests and problems. DSN is an improvement over earlier mechanisms for returning delivery status information. It can, for example, supply different status information for each recipient when multiple recipients are specified. It can also be used to generate return receipts on a per-recipient basis. DSN status is returned in the MIME encapsulated portion of a mail message's body.

DSN is defined in RFC1891, RFC1892, RFC1893, and RFC1894. If you wish to exclude DSN support (not recommended), you can turn it off with a line such as the following in your Build m4 file:

APPENDDEF(`confENVDEF', `-DDSN=0 )
                              
                    turn off DSN support

There is no debugging command-line switch to determine if DSN was defined for a precompiled version of sendmail. Instead, you must run sendmail with the -bs command-line switch and issue the EHLO SMTP command. If the following line shows up, it was defined:

250-DSN

If this line does not appear, check to see if noreceipts is defined for the PrivacyOptions option (See this section). If it was, you will have to undefine it for this line to appear. Otherwise, if this line does not appear, you will have to get either a new version of sendmail from your vendor, or open source sendmail and build it yourself.