I noticed that when you run
./discourse-setup again after an initial run, it seems to remember the values entered during the previous run (i.e. email addresses, smtp password etc). But it does not remember the SMTP user name, worse even, it remembers it wrongly.
It neither suggests the value that is actually in the app.yml nor does it resort to the default value (
firstname.lastname@example.org). Instead it seems to construct the suggested address by combining
postmaster@ with the forum’s domain (I think).
If your mail domain and your forum domain are the same, there is no problem. But it’s not uncommon to use a specific mail domain for mailgun & co, e.g.
mail.mydomain.com. The correct SMTP username in that case is
email@example.com (and that is what it says in the app.yml file. But the setup will use
firstname.lastname@example.org if your forum is at
mydomain.com. (Not sure if it will use
email@example.com if the forum is at
forum.mydomain.com, but either way it would be wrong.)