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

Postfix is an open-source Mail Transfer Agent that accepts, routes, and delivers email messages using the SMTP protocol, typically listening on port 25. It is one of the most common MTA deployments on Linux servers and acts as the core of a mail server infrastructure. The remaining choices serve completely unrelated roles and have no capability to handle email delivery at the server level.

Thunderbird is a desktop email client used to read and compose messages, not a server-side MTA.

Apache is an HTTP web server and has no functionality related to email routing or delivery.

GIMP is a raster graphics editor with no connection to email server functionality.

MySQL is a relational database management system and does not process or route email.