gitlab-org--gitlab-foss/config/initializers/action_mailer_hooks.rb

unless Gitlab.config.gitlab.email_enabled
  ActionMailer::Base.register_interceptor(::Gitlab::Email::Hook::DisableEmailInterceptor)
  ActionMailer::Base.logger = nil
end

ActionMailer::Base.register_interceptors(
  ::Gitlab::Email::Hook::AdditionalHeadersInterceptor,
  ::Gitlab::Email::Hook::EmailTemplateInterceptor,
  ::Gitlab::Email::Hook::DeliveryMetricsObserver
)

ActionMailer::Base.register_observer(::Gitlab::Email::Hook::DeliveryMetricsObserver)

if Gitlab.config.gitlab.email_enabled && Gitlab.config.gitlab.email_smime.enabled
  ActionMailer::Base.register_interceptor(::Gitlab::Email::Hook::SmimeSignatureInterceptor)
  Gitlab::AppLogger.debug "S/MIME signing of outgoing emails enabled"
end