thanks both for replying. In this case I was sure that it wasn't a
resolver timeout as the PTR is hosted on a local DNS server and failed
every time.
I just created a forward record for the server and its now all working
as expected, so thanks Mike for the tip! This has been confusing me
for a while as we've been asking for people to ensure they have a PTR
for proper mail delivery then still having issues with Exim!
