The main difference between Klaviyo, a marketing automation platform, and Mailjet, an email marketing platform, is in their scope and functionality. Klaviyo is a full-featured marketing automation solution designed to help eCommerce businesses grow their sales through targeted and personalized campaigns that leverage customer behavior data. In comparison, Mailjet is a simpler email marketing platform that is more focused on bulk email campaigns, newsletter management, and transactional emails. While both platforms provide email marketing tools, Klaviyo's capabilities extend beyond email marketing to include CRM, segmentation, automation, and analytics features that are not present in Mailjet. So, if you're looking for a more comprehensive marketing automation solution to drive your eCommerce business growth, Klaviyo may be the better option, but if you're looking for a simpler solution for managing email newsletters, then Mailjet may be more suitable.