Network interfaces can have one or more addresses associated with each interface, and each address will have a hostname associated with it. For example, on a machine with two interfaces, the one connected to the outside world might have the name host.your.domain, whereas the interface that is connected to the internal network might have the name host.sub.your.domain.

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 hostname associated with the local interface over which the outbound connection was made.

The ${if_name_out} macro is useful with the syslog database-map (syslog) for logging which interface was used to send messages. Note that a $& prefix is necessary when you reference this macro in rules (that is, use $&{if_name_out}, not ${if_name_out}).

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