fix(config): lots of SMTP servers do not accept localhost.localdomain

We need to force the `domain` in `app.yml`.

discourse/config/files/default/app.yml.jinja

@@ -61,6 +61,7 @@ env:
   ## on initial signup example 'user1@example.com,user2@example.com'
   DISCOURSE_DEVELOPER_EMAILS: {{ discourse.developer_emails }}
 
+  DISCOURSE_SMTP_DOMAIN: {{ discourse.hostname }}
   ## TODO: The SMTP mail server used to validate new accounts and send notifications
   # SMTP ADDRESS, username, and password are required
   # WARNING the char '#' in SMTP password can cause problems!