[exim] rDNS lookup problem

Hi,

I'm running Exim 4.86.2 on FreeBSD 9.3 and I'm having an issue whereby
it is registered incoming mails as originating from hosts without an
rDNS record, but they do have an rDNS record. I've just enabled the
X-Host-Lookup-Failed ACL to add the extra header and it confirms what I
was seeing by enterpreting the "Recieved:" header.

For example when sending from 93.191.32.251 I see:

X-Host-Lookup-Failed: Reverse DNS lookup failed for 93.191.32.251
(failed)

and

Received: from [93.191.32.251] (port=49828 helo=asigra2.ldex.co.uk)

but nslookup from the mail server itself provides:

nslookup 93.191.32.251
Server: 8.8.8.8
Address: 8.8.8.8#53

Non-authoritative answer:
251.32.191.93.in-addr.arpa name = asigra2.ldex.co.uk.

Any clues as to why Exim is failing when nslookup/dig from the same host
using the same DNS server work?

thanks in advance, Andy.