Set up Ghost with Mailgun on Google Conputer Engine

First please refer to :
https://docs.ghost.org/v1.0.0/docs/mail-config
As you may know, Google Cloud Platform doesn't allow outbound connection to ports 25, 465 and 587.
https://cloud.google.com/compute/docs/tutorials/sending-mail/
So you just change port of MailGun to other port such as 2525, it will work.
"mail": {
"transport": "SMTP",
"options": {
"service": "Mailgun",
"port": 2525,
"auth": {
"user": "yourname",
"pass": "yourpasswd"
}
}
},
When you have finished making changes to you config file don't forget to restart Ghost. ( ghost stop ghost start )