Automation of radiotherapy treatment planning improves efficiency and consistency, while reducing planning time and errors. The objective of this study was to validate an Iterative Optimization Engine (IOE) within an existing automated IMRT/VMAT planning framework. The IOE was designed to reduce remaining manual intervention within the automation framework through measuring and codifying common user intervention within a commercial planning system. The IOE was developed for external beam IMRT/VMAT treatment planning on the Monaco Treatment Planning System. The IOE was built on an existing automation framework, utilizing the Application Programming Interface (API) to create completely automated treatment plans. A network of 40 centers in Australia evaluated automated treatment plans for head and neck disease sites where users decided manual intervention was required post automated planning to achieve clinician approval. The modifications to automated plans were recorded, analyzed, and codified into the API to remove the requirement for manual intervention. A subset of the automated plans was then retrospectively processed by the IOE with resulting plans being scored in three categories of 1) superior, 2) equivalent and 3) inferior based on DVH assessment with the original clinician approved plan as the baseline. The automation framework generated 546 head and neck plans from January 1 to February 1, 2023, of which 45% required manual intervention to achieve dosimetric criteria. After being processed by the IOE, 86% of plans showed equivalent or superior coverage and maximum dose, and 95% of plans demonstrated equivalent homogeneity or improved homogeneity. Multi-target plans showed equivalent or improved target dose for 67% of intermediate dose targets and 39% of low dose targets when multiple targets were treated simultaneously. Analysis of organs at risk showed 38% of plans with reduced Parotid mean dose, 92% improved Larynx mean dose, 43% reduced Spinal Cord maximum dose, 57% decreased Brainstem maximum dose, 85% reduced Oral Cavity mean dose and 56% reduced Pharynx mean dose. The addition of an IOE achieved a clinical improvement to target and OAR metrics in the assessed clinical plans. The automation framework will incorporate this work into clinical production to improve the overall effectiveness of the automated planning framework.