sendmail, 4th Edition

Other than relaying mail via SMTP or LMTP, the sendmail program does not perform the actual delivery of mail. ^[1] 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.

^[1] For the purpose of this discussion, we gloss over the fact that sendmail actually can deliver directly to files (Section 12.2.2).