The check_data rule set can be used to validate recipients after all recipients have been declared with SMTP RCPT TO: commands. Other uses for the check_data rule set include screening a combination of sender and recipient, and evaluating connection-based information.

The check_data rule set is called from inside sendmail just after the SMTP DATA command is received, but before that command is acknowledged:

RCPT TO: <gw@wash.dc.gov>
250 2.1.5 <gw@wash.dc.gov>... Recipient ok
DATA
                                                    called here
354 Enter mail, end with "." on a line by itself    usually acknowledged here

The workspace passed to the check_data rule set is a count of the number of envelope recipients—that is, the number of accepted SMTP RCPT TO: commands. One use for this rule set might be to reject messages that specify too many envelope recipients. Consider the following mc configuration lines:

LOCAL_CONFIG
Kmath arith

LOCAL_RULESETS
Scheck_data
R $-            $: $(math l $@ 19 $@ $1 $)       # reject >=20 envelope recipients
R FALSE         $# error $@ 5.7.1 $: "550 Too many recipients"

Here, we add two new sections to our mc configuration file. The first, under LOCAL_CONFIG, defines a database map of type arith (arith).

In the second section, following the LOCAL_RULESETS, we declare the check_data rule set. That rule set is followed by two rules. The first rule looks up the workspace (the LHS $-) that contains a count of the envelope recipients. The RHS of that rule looks up that value using the math database map comparing the number of recipients (the $1 in the RHS) to a literal 19. If 19 is less than that count, FALSE is returned, indicating that there are too many recipients. Otherwise, TRUE is returned.

The second rule detects a literal FALSE (too many recipients) and uses the $#error delivery agent (error) to reject the message.

Note that we used 19 merely as an example. Before you decide on such a limit for your situation, you should consider how many users you have, and how many variations on those users there are in your aliases database. If you decide to limit the number of recipients you will accept, be sure that limit is large enough for all normal and mailing-list mail.