I checked the code quickly. I tries to send mails to every recipient one-by-one, with the request comes from Ghost. But sending mail with SMTP takes so much time. With a large recipient list, it can’t fit the timeout limit of Ghost (60 seconds, normally Mailgun responds under 1 second).
This is a confirmation of why sending bulk mail with SMTP is a bad idea. To achieve this, you need to have a distributed SMTP server, which can send mails concurrently from multiple servers at the same time. And you will need a queue mechanism to handle this in the background, while monitoring the failures.
These are the things Mailgun (or any other bulk mailing service) handle for you.
TLDR; don’t try to send newsletters with an SMTP server. At least use something like an AWS SES proxy, if you want to achieve a cheaper solution than Mailgun.