Mailshake is primarily an email marketing platform that focuses on sending personalized cold emails to prospects and generating leads. It offers features such as email tracking, campaign scheduling, and A/B testing. On the other hand, Klaviyo is a marketing automation platform that provides a complete marketing and sales automation toolset. It includes features such as segmentation, email marketing, personalized messaging, and advanced analytics. Klaviyo also integrates with eCommerce platforms to provide personalized product recommendations and targeted marketing campaigns based on customer behavior. Overall, Mailshake is more focused on outbound marketing efforts while Klaviyo is a complete marketing automation solution for businesses, especially those focused on eCommerce.