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37signals actually has a gem called mail_view that I've been using to preview mailers: https://github.com/37signals/mail_view

Glad this made it into Rails itself.

And if you've ever wanted to put inline CSS into emails, roadie will help with that: https://github.com/Mange/roadie

Hat-tip to roadie. I am absolutely amazed at how well it works, and how much work it removes in producing HTML emails.

The letter_opener gem is pretty handy to avoid that: https://github.com/ryanb/letter_opener

Or, you could easily just create controller actions which render your emails as well:

@body = mail.body.parts[1].html_part.body.raw_source

render '/mailer_notification/preview.html', :layout => false

And in preview.html.erb: <%= raw(@body) %>

I used mailcatcher. It's pretty awesome for this stuff, but glad to see it in core.