Domain rewriting rules, or, as they are frequently called, "rewrite rules," play two important roles. First, they are used to rewrite addresses into their proper or desired form. Second, they are used to determine to which channels a message should be enqueued. The determination of which channels to enqueue a message is made by rewriting its envelope To: addresses. Each rewrite rule appears on a single line in the upper half of the imta.cnf file.
For additional information about the domain rewriting process, refer to the Sun Internet Mail Server 3.5 Administrator's Guide.