When sendmail connects to another machine to send email, it gathers two pieces of information about that machine: its name and IP address. If the connection is over a network, the IP address is stored in this ${server_addr} macro. If the connection is local, as with LMTP, this ${server_addr} macro is given the name of the delivery agent as its value. If neither situation is true, this ${server_addr} macro is given a 0 (a literal zero character) as its value.

The ${server_addr} macro is used chiefly with the authinfo (Section 10.9.3.2), tls_server (Section 10.10.8.2), and try_tls (Section 10.10.8.4) rule sets.

The ${server_addr} macro is available for your use in rule sets, and can be useful, for example, in policy control. Note that a $& prefix is necessary when you reference this macro in rules (that is, use $&{server_addr}, not ${server_addr}).

${server_addr} is transient. If it is defined in the configuration file or in the command line, that definition can be ignored by sendmail. ${server_addr} must never be set with the macro database map (macro) to a value that is empty.