Mail test failure in discourse-doctor

Product Support
18

After a lot of testing, and a lot of help from @thoka, I offer the following summary:

I am able to run SMTP tests using the same creds that I have supplied container/app.yml that work with both swaks (Swaks - Swiss Army Knife for SMTP) and sample Ruby code provide by @thoka (see below).

These work consistently. However, trying to standup a Discourse server (which cannot send the initial email to the admin user) or using discourse-doctor bot result in email failures:

==================== MAIL TEST ====================
For a robust test, get an address from http://www.mail-tester.com/
Or just send a test message to yourself.
Email address for mail test? ('n' to skip) [admin@burlyqna.org]:
Sending mail to admin@burlyqna.org. . .
Testing sending to admin@burlyqna.org using smtp.mailfence.com:465, username:xxx@mailfence.com with plain auth.
SMTP server connection successful.
Sending to admin@burlyqna.org. . .
Sending mail failed.
end of file reached

This seems to be a protocol config failure, as I’m able send test emails using standard Ruby tools:

require 'mail'

SMTP_SERVER = 'smtp.mailfence.com'
SMTP_PORT = 465
USERNAME = 'xyz'
PASSWORD = '...'

FROM = 'admin@burlyqna.org'
TO = 'testing@gmail.com'
SUBJECT = 'Test from Ruby'

BODY = "Hello,\n\nThis is a test email sent from Ruby over a SSL-secured connection.\n\nBest regards!"

Mail.defaults do
  delivery_method :smtp, {
    address: SMTP_SERVER,
    port: SMTP_PORT,
    user_name: USERNAME,
    password: PASSWORD,
    domain: "burlyqna.org",
    enable_starttls_auto: :false,
    authentication: :plain,
    ssl: true,
  }
end

begin
  puts 'Sending email...'
  puts "SMTP_SERVER: #{SMTP_SERVER}"
  puts "SMTP_PORT: #{SMTP_PORT}"
  puts "USERNAME: #{USERNAME}"
  puts "PASSWORD: #{PASSWORD}"
  puts "FROM: #{FROM}"
  puts "TO: #{TO}"

  mail = Mail.new do
    to TO
    from FROM
    subject SUBJECT
    body BODY
  end

  mail.deliver!
end

The current SMTP settings in my app.yml are:

DISCOURSE_SMTP_ADDRESS: smtp.mailfence.com
DISCOURSE_SMTP_PORT: 465
DISCOURSE_SMTP_USER_NAME: xxx@mailfence.com
DISCOURSE_SMTP_PASSWORD: '...'
DISCOURSE_SMTP_FORCE_TLS: true                   # https://meta.discourse.org/t/cannot-send-email-problem-with-port-465/246004/8
DISCOURSE_SMTP_ENABLE_START_TLS: false           # (optional, default true)
DISCOURSE_SMTP_DOMAIN: burlyqna.org              # (required by some providers)
DISCOURSE_NOTIFICATION_EMAIL: noreply@talk.burlyqna.org