For quite awhile, ZMailer's scheduler has operated in such a way that if there is a new message going to Channel-C/Host-H, and the system has resources available to run a new (or recycled) transport agent, then appearance of the new job will run ALL accumulated jobs to that particular target at the same time.

Defining parameters:
    interval=1h
    retries="12"
would make sure that any such thread be retried only every 12 hours, although (as we said above) appearance of a new message will also cause run attempt.

Whatever, it will use up only one transport agent, which operates independent of all of the other message flows going around. That is, it will not cause significant slowdown to the system, not even when there are thousands of messages going to that one thread... (All this assuming you don't have too many of such targets with huge queues in your system -- like several dozens of them ...)

With 2.99.49p5 there is configuration flag "queueonly", which just accumulates the target thread(s), but will never automatically start the queueing. Instead, the target Host-H MUST be triggered by explicite "ETRN Host-H" command thru the smtpserver.

This will, of course, run all channels with Host-H. However, we thought that to be a minor nuisance, instead of demanding things like "it must be in 'smtp' channel" -- because we think it will be easiest to have explicite 'route' entry telling something like:

target.host    smtp-etrn!

and associated scheduler.conf target:

smtp-etrn/*
...
queueonly
command="smtp -c smtp-etrn ..."

For that route example, the necessary ETRN is:

ETRN target.host