Introduction
When you enable the Auto Expunge Trash option in Home » Service Configuration » Mailserver Configuration in WHM, the system automatically removes messages in the Trash folder during the nightly upcp run based on the value set in the Trash Expire Time field. Note that the mail must have been moved into the Trash folder after the Auto Expunge Trash was enabled.
Procedure
To manually clear out messages in the Trash folder before the nightly upcp run, set the value of Trash Expire Time to the desired value on the Mailserver Configuration page in WHM; the minimum value is 1, for messages that were already moved into the Trash at least one day ago. Then connect to your server via SSH and run this command as the root user:
/scripts/dovecot_maintenance