Steps to allow email sending on heroku through sidekiq.

config.rb

# Create Procfile which is required by heroku.

web: bin/rails server -p $PORT
worker: bundle exec sidekiq -c 2 -e staging

# REDIS
# REDISTOGO_URL for local is bydefault: redis://0.0.0.0:6379/0
# REDISTOGO_URL for heroku
# Use Redistogo heroku addon
# Addon will provide REDISTOGO_URL which should be added in the heroku config.

# config/initializers/sidekiq.rb
Sidekiq.configure_server do |config|
  config.redis = { url: ENV["REDISTOGO_URL"], id: nil }
end

Sidekiq.configure_client do |config|
  config.redis = { url: ENV["REDISTOGO_URL"], id: nil }
end

# smtp settings:
# https://guides.rubyonrails.org/action_mailer_basics.html#action-mailer-configuration-for-gmail
# make sure domain name does not contain https. 
config.action_mailer.smtp_settings = {
    address:              'smtp.gmail.com',
    port:                 587,
    domain:               ENV['DOMAIN'],
    user_name:            ENV['GMAIL_USERNAME'],
    password:             ENV['GMAIL_PASSWORD'],
    authentication:       'plain',
    enable_starttls_auto: true }

config.action_mailer.default_url_options = { :host => ENV['DOMAIN'] }

# And since we are using gmail therefore google blocks the application to connect with gmail. Therefore we need to allow that connection.
# Reference is here - https://stackoverflow.com/a/20262500/1468122

# Sidekiq docs from wiki
# Heroku - https://github.com/mperham/sidekiq/wiki/Heroku
# Redis - https://github.com/mperham/sidekiq/wiki/Using-Redis#multiple-redis-instances