Action Mailer, production mode, heroku and gmail - ruby-on-rails

Question

Action Mailer works for my app in development mode but not in production mode. I have pulled together a few possible solutions, none of which work for me.
production.rb:
require "active_support/core_ext/integer/time"
Rails.application.configure do
# Settings specified here will take precedence over those in config/application.rb.
# Code is not reloaded between requests.
config.cache_classes = true
# Eager load code on boot. This eager loads most of Rails and
# your application in memory, allowing both threaded web servers
# and those relying on copy on write to perform better.
# Rake tasks automatically ignore this option for performance.
config.eager_load = true
# Full error reports are disabled and caching is turned on.
config.consider_all_requests_local = false
# Ensures that a master key has been made available in either ENV["RAILS_MASTER_KEY"]
# or in config/master.key. This key is used to decrypt credentials (and other encrypted files).
# config.require_master_key = true
# Disable serving static files from the `/public` folder by default since
# Apache or NGINX already handles this.
config.public_file_server.enabled = ENV["RAILS_SERVE_STATIC_FILES"].present?
# Enable serving of images, stylesheets, and JavaScripts from an asset server.
# config.asset_host = "http://assets.example.com"
# Specifies the header that your server uses for sending files.
# config.action_dispatch.x_sendfile_header = "X-Sendfile" # for Apache
# config.action_dispatch.x_sendfile_header = "X-Accel-Redirect" # for NGINX
# Force all access to the app over SSL, use Strict-Transport-Security, and use secure cookies.
# config.force_ssl = true
# Include generic and useful information about system operation, but avoid logging too much
# information to avoid inadvertent exposure of personally identifiable information (PII).
config.log_level = :info
# Prepend all log lines with the following tags.
config.log_tags = [ :request_id ]
# Use a different cache store in production.
# config.cache_store = :mem_cache_store
# Enable locale fallbacks for I18n (makes lookups for any locale fall back to
# the I18n.default_locale when a translation cannot be found).
config.i18n.fallbacks = true
# Don't log any deprecations.
config.active_support.report_deprecations = false
# Use default logging formatter so that PID and timestamp are not suppressed.
config.log_formatter = ::Logger::Formatter.new
# Use a different logger for distributed setups.
# require "syslog/logger"
# config.logger = ActiveSupport::TaggedLogging.new(Syslog::Logger.new "app-name")
if ENV["RAILS_LOG_TO_STDOUT"].present?
logger = ActiveSupport::Logger.new(STDOUT)
logger.formatter = config.log_formatter
config.logger = ActiveSupport::TaggedLogging.new(logger)
end
# Do not dump schema after migrations.
config.active_record.dump_schema_after_migration = false
config.action_mailer.delivery_method = :smtp
config.action_mailer.smtp_settings = {
address: 'smtp.gmail.com',
port: 587,
domain: 'gmail.com',
user_name: ENV['gmail_username'],
password: ENV['gmail_password'],
authentication: 'plain',
enable_starttls_auto: true,
open_timeout: 5,
read_timeout: 5}
config.action_mailer.perform_deliveries = true
config.action_mailer.raise_delivery_errors = true
config.action_mailer.default_options = {from: 'xxxxx#gmail.com'}
config.action_mailer.default_url_options = { :host => 'MY-APP.herokuapp.com' }
end
I have tried everything here. I unlocked CAPTCHA on gmail by following this link: http://www.google.com/accounts/DisplayUnlockCaptcha. I have 2FA activated on my gmail account but am using an app-specific password.
How do I get Action Mailer to work in production on Heroku?