kumomta/docs/faq/flush_queue.md

How do I flush a queue?

{{since('2024.09.02-c5476b89')}}

You can use kcli rebind for this; the following will flush example.com for all tenant/campaigns:

$ kcli rebind --reason "Cleaning up a bad send" --domain example.com --always-flush

kcli rebind re-evaluates the queue for messages in matching queues. In the usage above, we're not passing in any new metadata, so the queue won't actually change. The --always-flush parameter tells KumoMTA that it should make the messages immediately eligible for delivery even though we didn't change the queue.

If you want to do this via API, then you should look at the /api/admin/rebind/v1 HTTP endpoint documentation.