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### 12.5 Why does Rmail need to write to `/var/spool/mail`?

This is the behavior of the `movemail` program which Rmail uses. This indicates that `movemail` is configured to use lock files.

RMS writes:

> Certain systems require lock files to interlock access to mail files. On these systems, `movemail` must write lock files, or you risk losing mail. You simply must arrange to let `movemail` write them.

> Other systems use the `flock` system call to interlock access. On these systems, you should configure `movemail` to use `flock`.