qmail-lspawn reads a series of local delivery commands from descriptor 0,
qmail-local to perform the deliveries,
and prints the results to descriptor 1.
qmail-local as the default delivery instruction.
so the results may not be in the same order as the commands.
For each recipient address,
qmail-lspawn finds out which local user controls that address.
It first checks the
qmail-users mechanism; if the address is not listed there, it invokes
qmail-lspawn then runs
qmail-local under the users uid and gid.
It does not set up any supplementary groups.
qmail-lspawn treats an empty mailbox name as a trash address.