Rails Mailer Sandbox 📬

Catch Rails emails locally before they escape into the wild. Rails Mailer Sandbox stores real .eml messages and gives you a Mailtrap-inspired inbox right inside your Rails app. No SMTP server, database, account, or monthly bill needed.

• Persistent MIME messages across Rails restarts
• Search by subject, sender, recipient, or full body
• HTML, plain-text, and raw-source views
• Desktop, tablet, and mobile preview widths
• From, To, Cc, Bcc, date, and message-size details
• Inline cid: image previews and attachment downloads
• UTF-8 attachment filenames
• Automatic inbox refresh without repeatedly parsing every email
• Individual and bulk deletion
• Sandboxed HTML previews with scripts and remote resources blocked
• Filesystem locking and atomic writes for Rails and background workers

Add gem to development group:

group :development do
  gem "rails-mailer-sandbox"
end

Set it up:

bundle install
bundle exec rake rails_mailer_sandbox:setup

Restart Rails, send any Action Mailer email, then visit:

👉 http://localhost:3000/rails_mailer_sandbox

Setup creates config/initializers/rails_mailer_sandbox.rb, configures Action Mailer, and mounts Web UI. Run task again safely; it will not overwrite edited initializer.

Prefer doing it yourself? Add this to config/environments/development.rb:

config.action_mailer.delivery_method = :rails_mailer_sandbox
config.action_mailer.perform_deliveries = true
config.action_mailer.raise_delivery_errors = true

Mount inbox in config/routes.rb:

mount RailsMailerSandbox::Server.new => "/rails_mailer_sandbox" if Rails.env.development?

Default path: Rails.root/tmp/rails_mailer_sandbox.

Want another location? Set it in initializer:

RailsMailerSandbox.storage_path = Rails.root.join("storage", "development_mail")

Keep custom storage out of version control. bin/rails tmp:clear removes default inbox. Filesystem storage targets local development on one host, not distributed deployments or giant email archives.

git clone https://github.com/azmi2409/rails-mailer-sandbox.git
cd rails-mailer-sandbox
bundle install
bundle exec ruby -Ilib test/rails_mailer_sandbox_test.rb
gem build rails-mailer-sandbox.gemspec

CI runs tests and gem build on Ruby 3.0, 3.1, 3.2, and 3.3. Found a bug or have a neat focused improvement? Read CONTRIBUTING.md, then open an issue or pull request. 🙌

Release notes live in CHANGELOG.md. 📝

Use this gem in development only. Captured emails may contain reset links, personal data, or other secrets. Never expose mounted inbox publicly or enable it in production.

MIT. Build useful things and test your emails. 🎉