Systemweit
echo net.ipv6.conf.all.disable_ipv6=1 > /etc/sysctl.d/disableipv6.conf
SSH
echo AddressFamily inet >> /etc/ssh/sshd_conf
Postfix
postconf -e inet_protocols=ipv4
echo net.ipv6.conf.all.disable_ipv6=1 > /etc/sysctl.d/disableipv6.conf
echo AddressFamily inet >> /etc/ssh/sshd_conf
postconf -e inet_protocols=ipv4