+PH/19 Exim expands the IPv6 address given to -bh to its full non-abbreviated
+ canonical form (as documented). However, after a host name lookup from
+ the IP address, check_host() was doing a simple string comparison with
+ addresses acquired from the DNS when checking that the found name did
+ have the original IP as one of its addresses. Since any found IPv6
+ addresses are likely to be in abbreviated form, the comparison could
+ fail. Luckily, there already exists a function for doing the comparison
+ by converting both addresses to binary, so now that is used instead of
+ the text comparison.
+