What is the best/easy way to validate an email address in ruby (on the server side)?
You could look whether or not it matches a regexp like the one used in this Rails validator:
validates_format_of :email,:with => /\A[^@\s]+@([^@\s]+\.)+[^@\s]+\z/
But if you use Devise, simply do:
validates_format_of :email,:with => Devise::email_regexp
Source: http://lindsaar.net/2008/4/14/tip-4-detecting-a-valid-email-address
Edit 1:
useful website for tests: http://www.rubular.com/