It looks like two different problems.
Google’s DNS may just help you to access the wild on Internet. It’s for access the web.
For email sending, I am not sure how it works.[quote=“gvalchca, post:3, topic:43592”]
its a local server for internal use only:
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You have set a DNS record for your SMTP server. How you prevent it from unauthorized access since you are exposing your username and claim it doesn’t need a password?