Emails are generated & queued up as dynamic jobs. Whereas cron jobs run at fixed schedules, the dynamic jobs can be scheduled on demand, allowing us to offload the generation of emails as well as the sending. This model also means emails will get sent out sooner - you may well have noticed that particularly in local testing emails now come out within seconds as opposed to having to wait up to 2 minutes for the email job to execute.
To clear the dynamic job queue, you can empty the following tables:
delete from QRTZ_DYNAMIC_TRIGGERS
delete from QRTZ_DYNAMIC_JOB_DETAILS
delete from QRTZ_DYNAMIC_FIRED_TRIGGERS
(n.b. I would make sure the JS is stopped before deleting from these tables)