Tentar:
DELETE FROM wp_mail_queue
WHERE message_id = (SELECT * FROM (SELECT message_id FROM wp_mail_queue ORDER BY id LIMIT 0, 1) queue_table)
ORDER BY wp_mail_queue.id
LIMIT 100
DELETE FROM wp_mail_queue
WHERE message_id = (SELECT * FROM (SELECT message_id FROM wp_mail_queue ORDER BY id LIMIT 0, 1) queue_table)
ORDER BY wp_mail_queue.id
LIMIT 100