email_collection.rb

class EmailCollection
  include Enumerable
  delegate :each, :to => :email_addresses

  def initialize(raw_email_addresses)
    @raw_email_addresses = raw_email_addresses
  end
  
  def valid_emails
    select {|e| e.match(email_regex) }
  end
  
  def invalid_emails
    email_addresses - valid_emails
  end
  
  def valid?
    invalid_emails.any?
  end
  
  private
  
  def email_addresses
    @raw_email_addresses.split(/[,\n]+/).reject(&:blank?).map(&:strip)
  end

  def email_regex
    email_name_regex  = '[A-Z0-9_\.%\+\-\']+'
    domain_head_regex = '(?:[A-Z0-9\-]+\.)+'
    domain_tld_regex  = '(?:[A-Z]{2,4})'
    /\A#{email_name_regex}@#{domain_head_regex}#{domain_tld_regex}\z/i
  end
end

invitation.rb

class Invitation < ActiveRecord::Base

  validate :email_addresses_are_valid

  def send_invitations
    parsed_emails.each do |email_address|
      InvitationMailer.invite(email_address, email_subject, email_body).deliver
    end
  end

  private

  def email_addresses
    @email_addresses ||= EmailCollection.new(email_addresses)
  end

  def email_addresses_are_valid
    if !email_addresses.valid?
      message  = email_addresses.invalid_emails.join(', ') + ' '
      message << email_addresses.invalid_emails.many? ? "are not valid email addresses" : "is not a valid email address"

      errors.add(:email_addresses, message)
    end
  end

end