How stop Rail 5 from logging contents of emails sent?

In production.rb, my Rails 5.2.6 app sets config.log_level = :info

However, when an email is sent, the logger level is somehow/somewhere set to :debug so the content of the email is logged.

How can I set the log level for ActionMailer to :info so it will stop sending the contents of email in the logs?

Here on SO I see a lot of conflicting advice, most of it pre-Rails 5, and most of it aimed at turning off logging altogether rather than setting the log level... but sometimes via setting ActionMailer::Base.logger, sometimes via config.action_mailer.log_level (which in rails 5.6 gives an error "undefined method `log_level=' for ActionMailer::Base:Class (NoMethodError)"

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