By default, ActionMailer uses the "mailers" queue, but this entry
was not included in the list of queues for Sidekiq to use.

For more details:

* https://github.com/plataformatec/devise/wiki/How-To:-Send-devise-emails-in-background-(Resque,-Sidekiq-and-Delayed::Job)
* http://guides.rubyonrails.org/active_job_basics.html