When sendmail first assigns a value to the ${client_name} macro (${client_name}), it also looks up the hostname of that connecting client with DNS. Table 21-9 shows the possible results of that lookup.

The ${client_resolve} macro is useful in the Local_check_rcpt (Section 7.1.3) and Local_check_mail (Section 7.1.2) rule sets. It can, for example, be used to accept mail only from machines whose hostname can be successfully looked up with DNS.

LOCAL_RULESETS
SLocal_check_mail
R$*                    $: $&{client_resolve}
ROK                    $@ Okay
RTEMP                  $#error $@ 4.7.1 $: "450 Can't resolve hostname just now."
R$*                    $#error $@ 5.7.1 $: "550 Sending hostname must resolve!"

Here, the first rule transfers the value of ${client_resolve} into the workspace. The $& prefix (Section 21.5.3) prevents that macro from wrongly being expanded when the configuration file is read. The second rule accepts the address if it can be looked up. The third rule defers acceptance of any sender address that results in a temporary lookup error. The last rule bounces mail from any host that cannot be looked up, or that appears to be a forged address.

${client_resolve} is transient. If it is defined in the configuration file or in the command line, that definition can be ignored by sendmail. Note that ${client_resolve} is not guaranteed to be available in the check_compat rule set (Section 7.1.4). Also note that a $& prefix is necessary when you reference this macro in rules (that is, use $&{client_resolve}, not ${client_resolve}).