Validate email via smtp

smtp_email_validate.rb

class EmailValidation
  SMTP_PORT = 25
  CONNECTION_TIMEOUT = 3
  READ_TIMEOUT = 3
  REGEX_SMTP_ERROR_BODY_PATTERN = /(?=.*550)(?=.*(user|account|customer|mailbox)).*/i.freeze

  def initialize(verifier_domain:, verifier_email:)
    @verifier_domain, @verifier_email = verifier_domain, verifier_email
  end

  # Probe if email is valid on the remote mx
  # Largely salvaged from: https://github.com/rubygarage/truemail
  def smtp_valid?(email)
    punycode_email = punycode_encode(email)
    host = resolve_mx_exchange(email)

    # Look if server responds
    Timeout.timeout(CONNECTION_TIMEOUT) do
      TCPSocket.new(host, SMTP_PORT).close
    end

    # Then establish smtp-handshake
    connection = Net::SMTP.new(host, SMTP_PORT).tap do |settings|
      settings.open_timeout = CONNECTION_TIMEOUT
      settings.read_timeout = READ_TIMEOUT
    end

    responses = []
    connection.start do |request|
      responses << request.public_send(:helo, @verifier_domain)
      responses << request.public_send(:mailfrom, @verifier_email)
      responses << request.public_send(:rcptto, punycode_email)
    end

    # Verify all responses, to look for an error
    responses.none? { |response| response.match? REGEX_SMTP_ERROR_BODY_PATTERN }
  rescue Timeout::Error, EOFError => err
    # The remote server might hang up on us. Or we may not be able to reach it at all.
    # In those cases, the response is undetermined. We treat this as the address being
    # valid, even though we don't really know.
    true
  rescue RuntimeError => err
    false
  end

  def punycode_encode(email)
    return unless email.is_a?(String)
    return email if email.ascii_only?
    user, domain = email.split('@')
    "#{user}@#{SimpleIDN.to_ascii(domain.downcase)}"
  end

  def resolve_mx_exchange(email)
    email_address = Mail::Address.new(email)
    Resolv::DNS.open do |dns|
      dns.getresources(email_address.domain, Resolv::DNS::Resource::IN::MX).first.exchange.to_s
    end
  end
end