Sending Email is setup to use gmail over SSL, using a google App Password since Google's 2fa is enabled on my account.
I have set the "From Address" to "Webmin@mydomain.com" however the test email that I send to myself still uses the "Login" email from the "Authentication" section. "Webmin@mydomain.com" is registered as an email address with Google Domains and gmail does allow me to send emails from that address when it is set up as a sending account in gmail.com, so I know that gmail isn't blocking/rewritting the From address.