sendmail is a mail router. Mail from external hosts is routed to local users. Mail from local users is routed to the outside world. Mail from the user’s mail tool is routed to the correct mailer for delivery. In fact, all of the functions covered so far in this book—delivery, forwarding, relaying, and masquerading—are part of the work sendmail does as a mail router. The roles that the aliases database and the .forward file play in routing mail from one user account to another, and that relaying plays in routing mail from one host to another, are obviously components of mail routing. Even masquerading, which ensures that replies to the mail that originates from your site are routed to the correct system, is an aspect of routing.
Given that every recipe discussed so far has something to do with mail routing, why do we have a separate chapter with routing in the title? The reason is that the recipes in this chapter take mail routing and “BAM! Kick it up a notch!” This chapter focuses on sendmail features that provide increased control over mail routing.
sendmail routes mail to internal mailers based on the
mail’s delivery address. When sendmail processes a
delivery address, it returns a mail delivery triple that identifies
the mailer that will be used, the host to which the mail will be
sent, and the recipient address of the user to whom the mail is
bound. The result of this process can be seen with a
sendmail -bv email@example.com ...