class ActionMailer::Base

If the subject has interpolations, you can pass them through the +interpolations+ parameter.
humanized version of the action_name.
If it does not find a translation for the +subject+ under the specified scope it will default to a
Translates the +subject+ using Rails I18n class under [mailer_scope, action_name] scope.

def default_i18n_subject(interpolations = {})
  mailer_scope = self.class.mailer_name.tr('/', '.')
  I18n.t(:subject, interpolations.merge(scope: [mailer_scope, action_name], default: action_name.humanize))
end