How do I remove mail html content from Rails' logs?

You may also consider setting the log level of your whole application in production to something less verbose than :debug. That way you will still log info about the mails being sent, but without the content:

# config/environments/production.rb

Rails.application.configure do
  ...
  config.log_level = :info
  ...
end

ActionMailer::Base.logger = nil

or in your config/environments/{development,test,production}.rb, add :

Rails.application.configure do
  ...
  config.action_mailer.logger = nil
  ...
end