email.rb

class Email
  
  def initialize(address)
    @address = address
  end
  
  def valid?
    @address.match(email_regex)
  end
  
  def to_s
    @address
  end
  
  private
  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

email_parser.rb

class EmailParser

  def initialize(email_addresses)
    @email_addresses = email_addresses
  end

  def parsed
    @email_addresses.split(/[,\n]/).map {|email|
      email.blank? ? nil : Email.new(email.gsub(/\s/, ""))
    }.compact
  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 parsed_emails
    EmailParser.new(email_addresses).parsed
  end
  
  def email_addresses_are_valid
    if email_addresses.present?
      erroneous_emails = parsed_emails.select { |email|
        !email.valid?
      }
      errors.add(:email_addresses, error_message(erroneous_emails)) if erroneous_emails.any?
    end
  end
  
  def error_message(erroneous_emails)
    emails = erroneous_emails.join(", ")
    details = erroneous_emails.length == 1 ? "is not a valid email address" : "are not valid email addresses"
    "#{emails} #{details}"
  end

end