Mail merge is a technique that allows sending personalized emails to multiple recipients using a template, which is particularly useful for high-volume sends that would be cumbersome with manual copy-pasting. This guide demonstrates how to create a mail merge application using Java and Gmail, leveraging the Nylas Email API for efficient handling of personalized email communications. The setup involves creating a .csv file containing recipient details and configuring a Java environment with necessary libraries like SparkJava for web framework, OpenCSV for handling CSV files, and Mustache for templating. The core functionality is implemented in a Java class that reads the CSV file, replaces placeholders in the email template with actual data, and sends out personalized emails to each recipient using the Nylas Java SDK. The application provides a web interface to input email subjects, bodies, and the CSV file, ensuring all fields are filled before submission, and sends out emails upon form submission, displaying the list of email addresses that received the communication.