Prefer IPv4 routing over IPv6

If you have an IPv4 and and IPv6 default gateway, but you wish for mail to be routed via the IPv4 network instead of the IPv4 then create a file /etc/gai.conf which contains:

# cat /etc/gai.conf <<EOF
label       ::1/128        0
label       ::/0           1
label       2002::/16      2
label       ::/96          3
label       ::ffff:0:0/96  4
precedence  ::1/128        50
precedence  ::/0           40
precedence  2002::/16      30
precedence  ::/96          20
precedence  ::ffff:0:0/96  100

EOF