Can not specify different DNSBLs for IPv4/IPv6

There is only one hook for checking DNSBLs in acl/30_exim4-config_check_rcpt which is run unconditionally for both IPv4 and IPv6 addresses. This can lead to problems when IPv6 addresses are checked against IPv4-only lists.

From exim spec 40.35:

40.35 DNS lists and IPv6
If Exim is asked to do a dnslist lookup for an IPv6 address, it inverts it nibble by nibble. For example, if the calling host’s IP address is 3ffe:ffff:836f:0a00:000a:0800:200a:c031, Exim might look up

  1.3.0.c.a.0.0.2.0.0.8.0.a.0.0.0.0.0.a.0.f.6.3.8.f.f.f.f.e.f.f.3.blackholes.mail-abuse.org

Unfortunately, some of the DNS lists contain wildcard records, intended for IPv4, that interact badly with IPv6. For example, the DNS entry

  *.3.some.list.example. A 127.0.0.1

is probably intended to put the entire 3.0.0.0/8 IPv4 network on the list. Unfortunately, it also matches the entire 3::/4 IPv6 network.

You can exclude IPv6 addresses from DNS lookups by making use of a suitable condition condition, as in this example:

  deny condition = ${if isip4{$sender_host_address}}
          dnslists = some.list.example