How to force IPv4 connections for specific domains if IPv6 is enabled

This scenario arose as a result of the ISP (BTNET 21CN) providing IPv6 and then not being able to provide reverse PTR records for the IPv6 addresses. It turns out that RIPE has not given the ISP SOA on that IPv6 subnet yet:

;; AUTHORITY SECTION:
0.a.2.ip6.arpa.         3600    IN      SOA     pri.authdns.ripe.net. dns.ripe.net. 1407144119 3600 600 864000 7200

As a consequence, some IPv6 capable recipients reject e.g. the following:

2014-08-04 12:31:56 1XEGUa-0006Kb-N8 ** [email protected] R=dnslookup T=remote_smtp X=TLSv1.2:AES256-GCM-SHA384:256: SMTP error from remote mail server after RCPT TO:<[email protected]>: host mail-c.linkedin.com [2620:109:c006:104::215]: 554 5.7.1 The sending mail server at 2a00:xxxx:xxxx:0:xxx:xxx:xxxx:xxxx does not have a reverse (address-to-name) DNS entry cf http://en.wikipedia.org/wiki/Reverse_DNS_lookup

Domainlist

Add a domainlist:

domainlist ipv4_force_domains = \
        gmail.com : \
        googlemail.com : \
        virgin.net : \
        linkedin.com : \
        virginmedia.com

Router configuration

Add a router:

ipv4_only:
        driver = dnslookup
        domains = +ipv4_force_domains
        transport = ipv4_smtp
        ignore_target_hosts = <; 0::0/0

Transport configuration

Add a transport:

    ipv4_smtp:
        driver = smtp
        dkim_domain = mydomain.co.uk
        dkim_selector = x
        dkim_private_key = /usr/exim/dkim.private.key
        dkim_canon = relaxed
        interface = <my.v4.ip.address>