The recipients of an email message can be specified or added to the message in several ways:

• Recipients can be specified as part of sendmail's command line (Section 15.3).

• Recipients can be specified in message headers if a -t command-line argument is used with sendmail (-t).

• Recipients can be specified with the RCPT TO: command (MaxRecipientsPerMessage).

• Recipients can be added using aliasing, (Section 12.1), mailing lists (Section 13.1), and expansion of users' ~/.forward files (Section 13.7).

• The MILTER interface (Section 7.6) can add and remove recipients as a result of policy decisions.

As each recipient is added to the internal list of recipients, sendmail updates the ${nrcpts} macro to reflect the current count.

The ${nrcpts} macro can be useful in the check_compat rule set (Section 7.1.4), which is called just before delivery. The value in ${nrcpts} can be used to check the number of recipients, and to possibly refuse delivery if there are too many recipients. (See also the MaxRecipientsPerMessage option, MaxRecipientsPerMessage.)

${nrcpts} 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 $&{nrcpts}, not ${nrcpts}).