This message is about how to fix the common @@token sendmail errors. There are instances when you receive email bounce messages because of syntax errors complaining that it does not know how to send email to @@token. Probably a site is NOT running NIS and is generating these errors or is talking to another site that is generating the errors and then passing the email on to your site. This happens because a single token is changed into a null ("") token. As a result, ruleset 3 (S3) changes null tokens into @@token. There are two key issues here. First, you do not want to be the host responsible for generating these errors, and, second, you do not want to pass along any errors that were generated by other hosts.