The default sender in Ghost is SMTP or Mailgun. I like Mandrill better, so here is how you can use it in Ghost.

Insert the following into your config.js file:

production: { // or development
    mail: {
        transport: 'SMTP',
        host: 'smtp.mandrillapp.com',
        options: {
            service: 'Mandrill',
            auth: {
                user: 'your@username.com',
                pass: 'yourAPIKey'
            }
        }
    }
}