If the
originating hostname is an IP address in square brackets, sendmail attempts to convert the
number to a hostname. The final result, in the form user@host, is assigned to $_.
When sendmail is run on the local machine, it sets $_ to be the name of the user that corresponds
to the user-id of the process that ran sendmail. It gets that name by calling
getpwuid(3). If the call fails, the name is set to the string:
Unknown UID: num
Here, num is the user-id for which a login name could not be found.
Next, an @ and the name of the local machine are appended to the name, and the result is
assigned to $_.
Beginning withV8.7 sendmail, attempts at IP source routing can also be stored in this
macro. If sendmail was compiled withIP_SRCROUTE defined, that IP source routing
information will be added to $_ after the user and host described earlier. The format of this
additional information is described in ?§3.4.16 on page 116.
The $_ macro is used in the standard Received: header like this:
HReceived: $?sfrom $s $.$?_($?s$|from $.$_)$.
Note that the $& prefix is necessary when you reference this macro in rules (that is, use $&_,
not $_).
21.9.2 $a
The origin date in RFC2822 format All versions
The $a macro holds the origin date of a mail message (the date and time that the original
message was sent). It holds a date in ARPAnet format, defined in RFC2822, section 3.
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