The S= rule set follows rule set 1 and rewrites
the sender address; the
set follows rule set 2 and rewrites the recipient address
They are declared inside a delivery agent declaration like
When there is just a single set specified, it is the rule set used to rewrite both header and envelope addresses. When a slash is present, the rule set to the left rewrites the envelope and the one to the right, the headers. If any is missing or zero, that rewriting is skipped.
Any S= rule set may be expressed as:
When identified by name, a number is internally assigned by sendmail. An optional = and number may follow a name to force specific association of number to name.