Other than relaying mail via SMTP or LMTP, the sendmail program does not perform the actual delivery of mail. Instead, it calls other programs (called mail delivery agents) to perform that service. Because the mechanics of delivery can vary so widely from delivery agent to delivery agent, sendmail needs a great deal of information about each delivery agent. Each sendmail M configuration command defines a mail delivery agent and the information that sendmail needs.
 For the purpose of this discussion, we gloss over the fact that sendmail actually can deliver directly to files (Section 12.2.2).