In order for a reseller to use the "Mail Queue Manager," the reseller will need to have the "Everything (root)" permission set.
Permissions for resellers can be configured from WHM >> Home »Resellers »Edit Reseller Nameservers and Privileges.
For detailed information about reseller privileges, please visit:
https://docs.cpanel.net/whm/resellers/edit-reseller-nameservers-and-privileges/