qmail-start arranges for qmail-send's activity record to be sent to qmail-start's output. See qmail-log(5) for the format of the activity record. Other than this, qmail-start does not print anything, even on failure.
If defaultdelivery is supplied, qmail-start passes it to qmail-lspawn.
If logger is supplied, qmail-start invokes logger with the given arguments, and feeds qmail-send's activity record through logger.
Environment variables given to qmail-start will eventually be passed on to qmail-local, so make sure to clean up the environment if you run qmail-start manually:
# env - PATH="HOME/bin:$PATH"
qmail-start ./Mailbox splogger qmail &
(all on one line)
Resource limits, controlling ttys, et al. are also passed from qmail-start to qmail-local.
Note that qmail-send normally juggles several simultaneous deliveries. To reduce qmail-send's impact on other programs, you can run qmail-start with a low priority.