This is a simple way to install mailhog, might not be the best solution for everyone, you can look for repositories, still have to register the binary in the PATH, among other things, this is a manual simple install.
Download mailhog from the releases page on github: https://github.com/mailhog/MailHog/releases
Save the binary at /opt/mailhog/mailhog
Give it executable permission chmod +x /opt/mailhog/mailhog
Download the gist: https://gist.github.com/renatomefi/d133fea9cb5a7b00f91edb24b83d9a31#file-init-d-mailhog-sh
Put it at /etc/init.d/mailhog