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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.