The tutorial provides a step-by-step guide on building a Retool app that integrates with the SendGrid API to send emails to customers within an hour. It outlines the process of connecting SendGrid to Retool by creating and configuring an API key, then pulling a list of users from a Postgres database to populate a table. The app's user interface includes a rich text editor for composing emails, modals for editing email details and viewing recipients, and a button for sending emails. The guide details creating queries to format email content, make POST requests to SendGrid's API, and iterate through selected email addresses to send emails. It also suggests additional features like adding attachments, using BCC, and personalizing emails, emphasizing the app's adaptability and further potential with SendGrid and Retool.