Loop by MESA https://apps.shopify.com/mesa

With the Loop by MESA app, you can repeat the next step in your workflow for each value identified in the step, allowing fine-tuned control over when your workflow runs. 

For example, if you want to alert customers when a product goes out of stock, Loop by MESA will go through each product option to verify its status, then send an email/SMS. Or you can incentivize loyalty by segmenting out your top customers and offering them additional discounts through an email campaign.