Docs improvement suggestion:
When configuring SMTP with an external provider (I was using SendGrid) I was not receiving registration emails. Running discourse-doctor identified the error: Reason: 550 The from address does not match a verified Sender Identity.
While I had the top level domain (example.com) authenticated for sending in SendGrid, I had not yet authenticated the discourse subdomain (discourse.example.com) and so SendGrid was rejecting the API calls.
While this only took me 15 minutes or so to figure out, I feel it would be helpful to include an additional comment in app.yml and the install script that makes this clearer to reduce friction for new users installing Discourse in future. Also a direct reference to the troubleshooting page would be helpful.
Something like:
# Most SMTP providers will require an authenticated domain or authenticated email
# address to send emails. Please ensure you have authenticated your sending domain
# (example.com) , sub-domain (discourse.example.com) and the `notifications` email
# address with your SMTP provider before registering new users to ensure email deliverability.
#
# The `discourse-doctor` command can assist with testing your email configuration.
# See: https://meta.discourse.org/t/troubleshoot-email-on-a-new-discourse-install