Action Mailer Cache Delivery 
Enhances ActionMailer to support the :cache delivery method, which behaves like :test, except that the deliveries are marshalled to a temporary cache file, thus making them available to other processes.
You'll want to use this gem if you're testing with Selenium (or any other tool which distinct processes).
Installation
Add to Gemfile
gem 'action_mailer_cache_delivery'
Now run bundle install
Usage
Change delivery method in your config/environments/test.rb
config.action_mailer.delivery_method = :cache
You can optionally specify location for cache file.
config.action_mailer.cache_settings = { :location => "#{Rails.root}/tmp/mail.cache" }
To access the cached deliveries in another process, just do
ActionMailer::Base.cached_deliveries
Use with parallel_tests
If you use parallel_tests to run your tests in parallel, you may get unexpected errors like EOFError. If so, make sure cache files differ for processes. Change you config/environment/test.rb
config.action_mailer.cache_settings = { :location => "#{Rails.root}/tmp/cache/action_mailer_cache_delivery#{ENV['TEST_ENV_NUMBER']}.cache" }
Contributors
I tried to list there everyone who forked original version on GitHub. If you're not here, just send pull request.