delayed_job: SystemStackError, stack level too deep

This should be written in bold:

If you have an #enqueue method in your job model, you’ll get this error

Quick fix: just add encode_with with empty method body for Mailer, that psych will know how to serialize Mailer class.

class Mailer
...
  def encode_with coder
  end
...
end

Problem lines which hide this bug by ‘stack too deep’ in https://github.com/collectiveidea/delayed_job/blob/master/lib/delayed/yaml_ext.rb#L5 so line with:

require File.expand_path('../psych_ext', __FILE__)

is never calls but it’s needed when to_yaml use psych engine as default.

UPD.: In my dependency was systemu, which reverting YAML engine to psych after DelayedJobs required with ‘syck’ engine, so yaml_ext works correctly, just my bad with my dependencies:

carrierwave_direct => uuid => macaddr => systemu.