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11.5 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.