You can access this feature from your MSL website. Only Organisation Admins with permissions to send messages can access the Messaging Admin window.
Currently, content editors don't have the option to cancel an email in progress. To cancel an outgoing email, contact firstname.lastname@example.org immediately. Include the organisation, email subject, and ID if possible. We also recommend following up with a phone call to the MSL team for prompt assistance.
To stop a pre-scheduled message from sending, see How to Cancel the Delivery of a Scheduled Email Message.