How to redirect a subdomain which doesnt have a website set up?

I have a subdomain mail.domain.com which we only use for outgoing mail. I want to disable the default website (the apache directory listing) which appears when visiting mail.domain.com in a browser. Redirecting it to https://www.domain.com would be best I guess. How can I do this please?

You might need to configure your apache config directly and instead of pointing to a documentroot, replace it with Redirect 301.