Prior to V8.7 the
$o macro stored as its value a sequence of
characters, any one of which could be used to separate the components
of an address into tokens. That role has been taken over by the V8.7
and above OperatorChars option (OperatorChars).
For backward compatibility the $o macro is still
honored by V8.7 sendmail in preversion 7
configuration files (Section 17.5). Otherwise, it is
unused in version 7 and above configuration files.