Postfix is an open-source mail transfer agent (MTA) used to route and deliver email. It's an incredibly powerful and reliable software that is incredibly popular among the developer community. It's fast, secure, and easy to configure and maintain. It supports a wide range of features, including encryption, authentication, and anti-spam measures. Postfix also supports multiple domains and virtual hosting, making it an ideal choice for those looking to host their own email servers. Overall, Postfix is one of the most popular and reliable solutions for email routing and delivery.