Hello,
Background
I shifted my 2 WordPress blogs which are over a decade old yesterday to a Ghost droplet on digital ocean.
When I tried to send this article to all my 1300 + subscribers I am getting "failed to send"notification. 
API Key
I have entered my private API Key and the mailgun server name is correct
Also, I did test email sending to myself and it works, it only fails to send the bulk email.
Note -
I have earlier this month shifted my personal blog to Ghost and emails to members are sent without any issues from the same mailgun account. I am using the same API keys for the other blogs too. namely Bandra.info and Powai.info
Do I need to configure different mailgun servers for these two blogs?
UPDATE
Hello
One domain works and another doesn’t work.
I set up 3 domains. I am using ghost CMS. I have copy-pasted the API’s to my setting under ghost.
All domains have been successfully set up. Now the one with mg.manojnayak.com is working fine which had only 6 subscribers.
But it seems like Bulk email send isssue.
But Bandra.info which has 1350 subscribers is not working yet. How can it be that I am using the same API’s and the domain is verified but still bulk emails are not going.
I have another domain Powai.info with 2500 subscribers, which I will be trying in a day or two.
Hope you could help me resolve this query as the Ghost forum also has many such complaints. With the issue is with Mailgun or Ghost.
Do advise.
Thank you.