When running as a daemon and listening for
incoming connections, sendmail attempts to look
up the true identity of connecting users and hosts. When it can find
that information, it saves it in the $_
sendmail macro ($_).
When transporting mail to other hosts, sendmail
looks up the MX records for those hosts and connects to the MX
records when they are available. If MX records are not available,
sendmail connects to A or AAAA addresses.
For recipient and sender syslog lines, the
relay= syslog equate shows
the name of the corresponding receiving or sending host, followed by
the A address, or AAAA address, of that host (if there was one) in
square braces:
relay=root@other.site.edu [123.45.67.89]
If the sender is a local user, the login name and
localhost will appear in the
relay= syslog equate:
relay=bob@localhost
In summary, the relay= syslog
equate shows who really accepted or sent the
message.