The sendmail program can send SMTP email over one or more network interfaces, where each interface can have one or more addresses associated with it. When sendmail sends a network email message, it begins by connecting to a host on the network. Once that connection has been made (once the other site accepts the connection), sendmail records in the ${if_addr_out} macro the address associated with the interface over which it made that outbound connection. It then records the family to which that address belongs in the ${if_family_out} macro. The family is a text representation of an integer value that represents the family, as defined in sys/socket.h. A value of 2, for example, could represent the AF_INET family.

${if_family_out} is available for use in rule sets, and can be useful for rejecting spam or restricting connections to particular addresses. Note that a $& prefix is necessary when you reference this macro in rules (that is, use $&{if_family_out}, not ${if_family_out}).

${if_family_out} is transient. If it is defined in the configuration file or in the command line, that definition can be ignored by sendmail.