[Exim] Relay from host list fails sometimes

Hello,

I have the following hostlist in my configure file:

hostlist relay_from_hosts = host1@??? : host2@??? : host3@???

If the IP address of host2@??? cannot be resolved (e.g.
due to DNS problems), no user at host3@???(!) can use
my mail server as a relay.

It appears the following message:

  no IP address found for host host2@??? (during SMTP
                   connection from foo.bar (host3@???)
  H=foo.bar (host3@???) .... relay not permitted

But any user on host1@??? can use my mail server as a
smart host. It seems that exim-4.30 cancels the forward dns
lookup loop to find the ip address of host3, if another
host prior in the list cannot be resolved.

This problem (bug?) does not appear with exim-4.11.

How can I fix this problem?

Greetings,

Frank