Carsten Nielsen has created an amazing plugin for rails called email veracity. It takes all of the heavy lifting out of what can often be a problematic part of data validation, and takes it a step further than most.
Not only does email veracity remove the need for a convoluted regexp to match against the email string, but it will also check the domain records to ensure that the MX entry is currently accepting mail, wow!
Add this to the code on your model definition:
that single line does the domain check too! You can turn it off, apply a timeout, and/or a custom error message if you like too:
validates_email_veracity_of :email, :domain_check => false, :timeout => 5, :message => "is not valid"
It's available as a plugin so use piston, or the following command:
script/plugin install http://svn.savvica.com/public/plugins/validates_email_veracity_of