Make it possible to disable IPv6 for SMTP
If ipv6 is on, than its also on für smtp. But we get no static IPv6 by Deutsche Telekom AG without a expensive contract; but we get a static IPv4. With no static ipv6 we cant configure the AAAA-Record / PTR /RDNS . So our ipv6-Mails will classified as SPAM because we cant disallow to send mails via ipv6. But we will need ipv6 in the future for VPN-connections.