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.