Which of the following software packages is an e-mail server?

Postfix is a popular open-source Mail Transfer Agent (MTA) that handles sending, receiving, and routing email on Linux servers. It operates as a server-side daemon, listening for SMTP connections and managing mail queues, making it a true email server application.

Thunderbird is a desktop email client used to read and compose email, not a server that routes or delivers mail.

Apache is an HTTP web server used to serve web pages over HTTP/HTTPS, not an email server.

GIMP (GNU Image Manipulation Program) is a graphics editor with no email server functionality.

MySQL is a relational database management system used for data storage, not for handling email delivery.