To add custom mail provider to your ghost config.js file, you can get hints from nodemailer documentation. Sample usage is here as follows (I use gandi.net webmail).

production: {
...
    mail: {
        transport: 'SMTP',
        from: '"Çağrı Çelebi" <***@cagricelebi.com>',
        options: {
            host: 'mail.gandi.net',
            port: 465,
            secureConnection: true,
            auth: {
                user: '***@cagricelebi.com',
                pass: '************'
            }
        }
    },
...

Hope it helps.