Currently IPv4 is preferred, according to the exim configuration:
/var/chroot-smtp/etc/exim.conf
disable_ipv6 = IPV6_DISABLE
prefer_ipv4 = IPV6_PREFER_IPV4
/var/chroot-smtp/etc/exim.conf.includes/macros
IPV6_DISABLE=false
IPV6_PREFER_IPV4=true
This should be changed in dependency to "Interfaces & Routing => IPv6"
If "IPv6 status" is on, then /var/chroot-smtp/etc/exim.conf.includes/macros should include
IPV6_PREFER_IPV4=false