Config file permissions allow viewing mail credentials

I am using Mailgun to send mail, which is working great.
On Ubuntu 18.04 I’ve installed exim4 and all email is routed to
The mailgun login and password are contained in this file:
-rw-r----- 1 root Debian-exim 283 Aug 18 22:58 passwd.client

In the case of ghost, is there simply an option to use the system’s MTA? This would be convenient since if the configuration changes I won’t have to remember to change it in multiple places.

But I modified the ghost config file to use mailgun, and that works fine. However, the mailgun credentials are in that file, and it appears impossible to alter the permissions/ownership on that file, as was done in the case of the exim4 file.

Is there some way to make this more secure?
Since ghost is already requiring root, why can’t you set the file like this:
-rw------- 1 ghostuser ghostuser 771 Aug 19 13:45 config.production.json

Okay, I discovered that I could set it to
“mail”: {
“transport”: “SMTP”,
“options”: {
“service”: “Sendmail”
and that resolves the issue in my case.