Prepare address assignments for qmail-lspawn
qmail-newu
qmail-newu reads the assignments in /var/lib/qmail/users/assign and writes them into /var/lib/qmail/users/cdb in a binary format suited for quick access by qmail-lspawn.
If there is a problem with users/assign, qmail-newu complains and leaves users/cdb alone.
qmail-newu ensures that users/cdb is updated atomically, so qmail-lspawn never has to wait for qmail-newu to finish. However, qmail-newu makes no attempt to protect against two simultaneous updates of users/cdb.
The binary users/cdb format is portable across machines.