Which of the following software packages is a mail server?

Postfix is a Mail Transfer Agent (MTA) designed to receive, route, and deliver email messages on a server. It listens for SMTP connections, processes mail queues, and delivers messages to local mailboxes or forwards them to other mail servers, fulfilling the role of a mail server. It is one of the most common MTAs found on Linux systems, alongside Sendmail and Exim.

Thunderbird is a mail client (MUA - Mail User Agent) used to read and compose email on a desktop, not a mail server that routes or delivers messages.

Apache is an HTTP web server used to serve web pages over the internet, not a mail server.

GIMP (GNU Image Manipulation Program) is a graphical image editor, entirely unrelated to email or mail services.

MySQL is a relational database management system used for storing and querying structured data, not a mail server.