The local mail store (where messages would be saved until the recipient came along to read them) was not formally standardized. Some sites liked to store them in a centralized place, such as /usr/mail, /var/mail, or /var/spool/mail. Other sites liked to store them in the recipient's home directory (e.g., as a file called .mail). Most sites started each message with a line beginning "From" followed by a space character (an extraordinarily bad decision, but that was the convention at the time), but sites that were Arpanet-focused usually stored messages separated by a line containing four control-A characters. Some sites attempted to lock the mailbox to prevent collisions, but they used different locking conventions (file locking primitives were not yet available). In short, the only reasonable thing to do was treat local mail storage as a black box.
On nearly all sites, the actual mechanism for doing local mailbox storage was embodied in the /bin/mail program. This had a (quite primitive) user interface, routing, and storage built into one program. To incorporate sendmail, the routing portion was pulled out and replaced with a call to sendmail. A -d flag was added to force final delivery, i.e., it prevented /bin/mail from calling sendmail to do the routing. In later years the code used to deliver a message to a physical mailbox was extracted into another program called mail.local. The /bin/mail program exists today only to include a lowest common denominator for scripts to send mail.