Did the error message provide more information apart from "you can't save the file"? Could you please share a screenshot of the error message when receiving that error message again?
I'm glad to hear that you manage to mount the volume and edit the config properly. If you already followed the troubleshooting section, we have an SMTP Checker: It will analyse your SMTP configuration in your application (currently supported apps: wordpress & redmine). It will show which SMTP configuration your application is using and whether the connection with the SMTP server is good and will also check the mail delivery.
Please download the latest version of Mac OS X from https://github.com/bitnami-labs/healthcheck-tools/releases/ (download the darwin binary)
Since your application is not supported yet, please find out your SMTP settings and run the command:
chmod +x ./smtp-checker-v2.0.0-darwin-amd64
./smtp-checker-v2.0.0-darwin-amd64 -smtp_host YOUR_SMTP_HOST -smtp_port YOUR_SMTP_PORT -smtp_user YOUR_SMTP_USER -smtp_password YOUR_SMTP_PASS -mail_recipient YOUR_MAIL_ACCOUNT
Please note that you need to replace the placeholders YOUR_SMTP_HOST, YOUR_SMTP_PORT, YOUR_SMTP_USER, YOUR_SMTP_PASS and YOUR_MAIL_ACCOUNT with the actual values you want to use.
Please show us the results of executing the commands.