11.2 Rule Set S=

In the original hub delivery agent definition the S= equate was set to zero:

Mhub,   P=[IPC], S=0, R=0, F=mDFMuXa, T=DNS/RFC822/SMTP, A=IPC $h
                   
           no custom processing

The S= equate is used to specify the number of the rule set that will perform delivery agent-specific custom processing of the sender's address. When that equate specifies rule set 0, sendmail does no custom processing. Now that we intend to do custom sender address processing, we will change the 0 into of a real rule set number and write that rule set.

Recall that rule sets 0 through 5 have special internal meanings to sendmail (see Section 8.3, "Rule Sets"). When selecting a rule number for use in a delivery agent's S= equate, you need either to carefully select one that does not already have internal meaning for sendmail or to let sendmail select the number for you. For the purposes of the client.cf file we will use the latter approach. [3]

[3] Available only beginning with V8.7 sendmail.

As the first step in creating a new rule set, edit the client.cf file and change S=0 to S=Hubset in the hub delivery agent definition.

Mhub,   P=[IPC], S=Hubset, R=0, F=mDFMuXa, T=DNS/RFC822/SMTP, A=IPC $h
                   
           change to symbolic name

This change tells sendmail to assign a number of its own choosing to S=. That rule set, which we will refer to symbolically by the name Hubset, will do a custom rewrite of the sender's address for this (the hub) delivery agent. But before you can create that actual rule set, we need to review why it is necessary.