RailsEmail

A polished mailer preview UI for Rails — inspired by React Email. Drop it in and your /rails/mailers preview route gets a modern dark/light interface with viewport controls, source inspection, CSS compatibility checks, and spam scoring.

Zero configuration. No build step. Works with your existing ActionMailer::Preview classes.

More screenshots

Source view	Mobile viewport

Compatibility checker

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Features

• Dark & light theme — toggle anytime, preference saved in localStorage
• Dark mode email rendering — LCH-based color inversion (same algorithm as React Email)
• Viewport presets — Mobile (375px), Tablet (768px), Desktop (1280px), Email (600px), or custom
• Source viewer — toggle between rendered preview and raw HTML / plain-text source
• Headers panel — view From, To, Subject, Date, Reply-To, CC, BCC at a glance
• CSS compatibility checker — detects unsupported properties (Flexbox, Grid, CSS variables, animations…) with the affected clients listed
• Spam score — runs your email through Postmark's SpamAssassin API and shows a score out of 10
• Download as .eml — grab the full RFC 822 message for local testing
• Locale selector — switch preview locale without leaving the page
• Collapsible sidebar — more room for your email when you need it

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Requirements

• Ruby >= 3.1
• Rails >= 7.0
• ActionMailer previews enabled (config.action_mailer.show_previews = true in development)

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Installation

Add to your Gemfile:

gem "rails_email"

Then run:

bundle install

That's it. No generators, no initializers, no configuration files needed.

Note: Make sure config.action_mailer.show_previews = true is set in config/environments/development.rb. Rails enables this by default in development, so you probably don't need to touch anything.

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Usage

Start your Rails server and visit:

http://localhost:3000/rails/mailers

Your existing ActionMailer::Preview classes appear in the left sidebar. Click any email action to open the preview.

Preview classes

RailsEmail works with standard ActionMailer previews. Place them in test/mailers/previews/ (or wherever config.action_mailer.preview_paths points):

# test/mailers/previews/user_mailer_preview.rb
class UserMailerPreview < ActionMailer::Preview
  def welcome_email
    UserMailer.with(user: User.first).welcome_email
  end

  def password_reset
    UserMailer.with(user: User.first).password_reset
  end
end

Spam check

The Spam tab sends the full RFC 822 message (headers + body) to Postmark's SpamAssassin API. No API key required. An internet connection is needed when running the check.

The score is displayed as X / 10 (higher is better). The individual SpamAssassin rules that fired are shown below the score, sorted by impact.

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Tabs reference

Tab	What it shows
Headers	From, To, Subject, Date, Reply-To, CC, BCC; locale switcher
Compatibility	CSS properties in the email that are unsupported by major clients
Spam	SpamAssassin score and the rules that contributed to it
Attachments	Files attached to the email with download links

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Keyboard / UI controls

Control	Action
Sidebar toggle (☰)	Collapse / expand the mailer list
Preview / Source buttons	Switch between rendered and raw views
Desktop / Mobile buttons	Switch between full-width and 375 px mobile viewport
Viewport dropdown	Fine-grained presets: Mobile, Tablet, Desktop, Email (600 px), Custom
Theme toggle (☀/☾)	Switch dark ↔ light; saved across sessions
↓ Download	Download the email as an .eml file
∨ (panel header)	Collapse / expand the bottom info panel

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How it works

RailsEmail is a Rails Engine that:

1. Prepends its own views to Rails::MailersController, replacing the default preview templates without touching the controller itself.
2. Adds a single new route — GET /rails/mailers/spam_check/*path — prepended before ActionMailer's wildcard so it is matched first.
3. Includes a controller extension that handles the spam check request: it calls your preview, serialises the result to a full RFC 822 string via email.to_s, and forwards it to Postmark's SpamAssassin API over TLS.

No monkey-patching, no middleware, no asset pipeline involvement. Everything is rendered as inline ERB (CSS and JS embedded in partials).

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Development

Clone the repo and install dependencies:

git clone https://github.com/your-org/rails_email.git
cd rails_email
bundle install

The test/dummy directory contains a minimal Rails app with sample mailers. Run it with:

cd test/dummy
bundle exec rails server

Then open http://localhost:3000/rails/mailers.

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Contributing

1. Fork the repository
2. Create a feature branch (git checkout -b my-feature)
3. Commit your changes (git commit -m 'Add my feature')
4. Push to the branch (git push origin my-feature)
5. Open a pull request

Please keep changes focused. Bug fixes and improvements to existing features are welcome. For larger changes, open an issue first to discuss the approach.

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License

MIT. See LICENSE for details.