I'm the author of the Rails wrapper for the big chunk of regex code; you're correct, it is for use cases that are akin to an MUA/MTA.

http://github.com/sixarm/sixarm_ruby_email_address_validatio...

We use a combination of client-side JavaScript validation and server-side validation in Rails. Typical server-side validation is for REST JSON API calls by third-party apps, and also for parsing freeform text fields like "tell your friends about us".

Original code is by Tim Fletcher & Cal Henderson.