SendGrid is an email marketing platform that focuses on sending 1-to-1 emails. It allows businesses to create automated, personalized email campaigns that can be used to nurture leads, promote products and services, and re-engage inactive customers. Mailshake is also an email marketing platform, however it specializes in 1-to-many emails that are designed for broadcast efforts. Mailshake is for big campaigns, such as newsletters, announcements, and promotions. It does not have the same level of personalization available as SendGrid, and is better suited for those who need to send large numbers of emails in a very short period of time.