mailing_job.rbshould be in a place where Rails can find and automatically download it. Where do you have this file? If it is not in app/modelsor something like that, it will not find it on its own.
I have my tasks in app/jobs, but it works because I added this path to the Rails load path in the block environment.rb Rails::Initializer.run:
config.load_paths << "#{Rails.root}/app/jobs"
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