`rake emails:test` task was always sending `localhost` as the domain name rather than using `smtp[:domain]` (aka `DISCOURSE_SMTP_DOMAIN`. `discourse-setup` has recently been modified to always set `DISCOURSE_SMTP_DOMAIN`, so it's important that this test rake task actually use the value.
I tested this change on a standard production install, and it's working as expected. Hopefully I managed not to bungle the copy/paste of the single line here in the github edit window.
Co-authored-by: Jay Pfaffman <jay@literatecomputing.com>