For the “From” address in all the emails sent from discourse, The setting I mentioned above is responsible. Your test@domainname.com SMTP username should be able to send on behalf of <somethingelse>@domainname.com for this to work. This is true by default on ESPs like mailgun. If you’re using some other email service, you might have to do extra config at the email server first.
And rebuild will only change the setting that you modify in app.yml everything else stays the same.