PMTA
How to Clear or Delete PMTA Queue

PowerMTA provides more reliable, scalable, and flexible sending than any other MTA. Sending high-volume, business-critical email is made easy with PowerMTA, an enterprise-grade message transfer agent.
There is a simple command to clear or delete PMTA queue. Just run:
pmta delete --queue=*/*
You should get following response:

Also, you can run all of these 3 commands together that will clear queue, reload configuration and restart PMTA.
pmta delete --queue=*/*
service pmta reload
service pmta restart
You should get following response:

At this time your PMTA is ready for the next batch of mailing.
The reliability, scalability, and flexibility of PMTA make it superior to other MTAs. A PMTA is ideal for sending high-volume, business-critical emails.