I've been using the validates_email_format_of gem for very very long time.

• Github: https://github.com/validates-email-format-of/validates_email_format_of
• Rubygems: https://rubygems.org/gems/validates_email_format_of

How does it compare?

• Validates against RFC 2822 and RFC 3696
• Does an actual DNS request, see https://github.com/validates-email-format-of/validates_email_format_of/blob/master/lib/validates_email_format_of.rb#L16-L18
• Has an rspec matcher https://github.com/validates-email-format-of/validates_email_format_of/blob/master/lib/validates_email_format_of/rspec_matcher.rb (I opened an issue for this here: Add rspec matcher #82)
• Provides some locales for some languages https://github.com/validates-email-format-of/validates_email_format_of/tree/master/config/locales