[Exim] relay question

Hi!

we are trying to configure "exim" like gateway for the mails in inbound and outbound from our internal-mail server and some local-server.
We have found in to configuration file the following parameter:

# If this is uncommented, we accept and relay mail for all domains we are
# in the DNS as an MX for.

#relay_domains_include_local_mx = true

Our Exim use external-DNS to solve domains, because our policy don't allow that the Internal-DNS can't solve Internet domains.
When we try to uncomment the parameter don't work fine.
We have found this error message:

Exim configuration error in line 97:
main option "relay_domains_include_local_mx" unknown

Our version of Exim is 4.20.

How can solve it?

Andrea Balzi