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.