When sendmail receives a message via SMTP, it can also receive information about how it should handle a bounce. That information is included as part of an envelope-sender declaration:

RCPT To:<address> NOTIFY=how

Here, the RCPT TO: command specifies an envelope recipient's address. Following that address is the keyword NOTIFY=, followed by one or more of four possible keywords: success, failure, never, and delay (see -N for a more complete description of NOTIFY= and its keywords).

The keywords specified are made the value of the ${dsn_notify} macro. If no NOTIFY= is specified, the ${dsn_notify} macro is undefined (NULL). If multiple RCPT TO: commands are issued during a single SMTP session, each command will update the ${dsn_notify} macro in turn, overwriting the prior RCPT TO: command's value.

The ${dsn_notify} macro is also given a value if the -N command-line switch (-N) is used to set the NOTIFY= keyword during mail submission.

One use for this ${dsn_notify} macro might be to log every instance when notification of success is requested. One way to do this is with a syslog map in the check_compat rule set:

LOCAL_CONFIG
Klog syslog -D -LNOTICE

LOCAL_RULESETS
Scheck_compat
R$*                     $: $&{dsn_notify} $| $1
Rsuccess $| $* $| $*    $: $(log dsn=success, recipient=$2, sender=$1 $)

Here, we declare a syslog map (syslog) with the K configuration command (Section 23.2) in the LOCAL_CONFIG part of your mc file. The -D tells sendmail to not syslog if the message is being deferred. The -L configuration command tells sendmail to syslog at level LOG_NOTICE (Section 14.3.1).

The LOCAL_RULESETS part of your mc file declares the check_compat (Section 7.1.4) rule set, which is called just after the check for too large a size (as defined by M=, M=). The workspace passed to check_compat is the sender and recipient addresses separated by a $| operator. The first rule simply places the value in the ${dsn_notify} macro at the beginning of the workspace and separates that value from the rest of the workspace with another $| operator.

The second rule looks for the success keyword. If it is found, the log map is called to syslog the three pieces of information shown.

${dsn_notify} is transient. If it is defined in the configuration file or in the command line, that definition can be ignored by sendmail. Note that a $& prefix is necessary when you reference this macro in rules (that is, use $&{dsn_notify}, not ${dsn_notify}).