letter_opener[1] is really useful to avoid the test account + wait situation. Well, now you can just use mailer previews, I guess :)

[1]: https://github.com/ryanb/letter_opener

Also Mailcatcher, which runs a local SMTP server with a nice email-client-style web UI: http://mailcatcher.me

premailer is also useful. Even with the new Rails 4.1 email previews (available in development env), sometimes it's useful to be able to preview in production. https://github.com/fphilipe/premailer-rails

How will it 'preview' the sent mail? E-mail clients behave differently when it comes to layout. When I'm doing such testing, I still verify the mail in at least the top 90% tier e-mail clients (web and desktop).

You're supposed to do the initial testing on the 'preview' and then actually test on real email clients.

Colors, fontsizes, margins, etc. on preview to get it right - then email clients to perfect it.

Unless you really need to be checking network deliver that much, mailers have long been testable, getting the last email sent from ActionMailer::Base.deliveries.last

But you can just use 'letter_opener' and pow: Emails open up instantly on the browser to preview.