email_blacklist_domain.rb

# app/models/email_blacklist_domain.rb
class EmailBlacklistDomain < ActiveRecord::Base
  validates :value, presence: true, uniqueness: { case_sensitive: false }
end

email_blacklist_ip_range.rb

# app/models/email_blacklist_ip_range.rb
class EmailBlacklistIpRange < ActiveRecord::Base
  validates :value, presence: true, uniqueness: true, ip_addr: true
end

email_validator.rb

# app/validators/email_validator.rb
class EmailValidator < ActiveModel::EachValidator

  def validate_each(record, attribute, value)
    result = false
    # change to /\A[^@\s]+@(([^@.\s]+\.)*[^@.\s]+)\z/ to allow local hosts to work
    if value =~ /\A[^@\s]+@(([^@.\s]+\.)+[^@.\s]+)\z/
      domain = $2
      result = dns_domain_exists?(domain) && ! forbidden?(domain)
    end
    record.errors[attribute] << (options[:message] || 'is not a valid email address') unless result
  end

protected

  def forbidden?(domain)
    forbidden_dns_domain?(domain) || forbidden_any_ip?(domain)
  end

  def dns_domain_records(domain)
    Resolv::DNS.open do |dns|
      dns_records = dns.getresources(domain, Resolv::DNS::Resource::IN::ANY)
      yield(dns_records)
    end
  end

  def dns_domain_exists?(domain)
    dns_domain_records(domain) do |dns_records|
      return true
    end
    return false
  end

  def forbidden_dns_domain?(domain)
    domain = domain.to_s
    EmailBlacklistDomain.all.each { |email_blacklist_domain|
      if domain =~ /\A.*#{email_blacklist_domain.value}.*\z/i
        return true
      end
    } 
    return false
  end

  def forbidden_any_ip?(domain)
    dns_domain_records(domain) do |dns_records|
      dns_records.map { |dns_record|
        next unless dns_record.respond_to? :address
        return true if forbidden_ip?(dns_record.address)
      }
    end
    return false
  end

  def forbidden_ip?(ip)
    EmailBlacklistIpRange.all.each { |email_blacklist_ip_record|
      begin
        value = email_blacklist_ip_record.value
        return true if IPAddr.new(value).include? ip.to_s
      rescue IPAddr::InvalidAddressError
        logger.warn "invalid ip addr or ip range in email_blacklist_ip table: #{email_blacklist_ip_record}"
      end
    }
    return false
  end
end

install.md

bundle exec rails g migration CreateEmailBlacklistDomains value:string:uniq notes:string
bundle exec rails g migration CreateEmailBlacklistIpRanges value:string:uniq notes:string
bundle exec rake db:migrate

ip_addr_validator.rb

# app/validators/ip_addr_validator.rb
require 'ipaddr'

class IpAddrValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    IPAddr.new value
  rescue IPAddr::InvalidAddressError
    record.errors[attribute] << (options[:message] || 'is not a valid IP (v4/v6) addresss or range')
  end
end

seeds.rb

# db/seeds.rb

### append the following to your seeds.rb

# EmailBlacklistIPRange
[
# mailinator.com
'207.198.106.56', 
# yopmail
'213.251.128.128/28', 
'213.251.188.128/28',
'2001:41d0:1:1980::1/60',
'2001:41d0:1:4a80::1/60',
].map { |value| EmailBlacklistIpRange.create! value: value }

# EmailBlacklistDomain
%w[
deadaddress\.com
sharklasers\.com
mailinator\.com
yopmail\.com
].map { |value| EmailBlacklistDomain.create! value: value }