Qmail is an open-source mail transfer agent (MTA) designed for sending and receiving email. Created by Daniel J. Bernstein in the late 1990s, it focuses on security, performance, and simplicity. Qmail is known for its modular architecture, allowing users to customize and extend its functionality easily. One of the key features of Qmail is its ability to handle large volumes of email efficiently while minimizing the risk of spam and other security threats. It supports various protocols, including SMTP and POP3, making it compatible with many email clients and services.