When changing code in mailers, updating the corresponding mailer preview is easily forgotten.

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Mailer previews can be tested like other code as well and I sometimes add the following test to test suites:

# Make sure to require the previews
Dir[Rails.root.join('test/mailers/previews/*.rb')].sort.each { |file| require(file) }


ActionMailer::Preview.all.index_with(&:emails).each do |preview, mails|
  mails.each do |mail|
    it "#{preview}##{mail} works" do
      expect { preview.call(mail) }.not_to raise_error
    end
  end
end

This reminder solves the problem of broken mailer previews.