Mhub, P=[IPC], S=0, R=0, F=mDFMuXa, T=DNS/RFC822/SMTP, A=IPC $h no custom processing
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
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.
 Available only beginning with V8.7 sendmail.
As the first step in creating a new rule set, edit the client.cf
file and change
hub delivery agent definition.
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
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.