postfix: Use IPv6 only for all usual MTAs to conncet to MX

Signed-off-by: Maximilian Wilhelm <max@sdn.clinic>

postfix/main.cf

@@ -39,3 +39,5 @@ mailbox_command = procmail -a "$EXTENSION"
 mailbox_size_limit = 0
 recipient_delimiter = +
 inet_interfaces = all
+# IPv6 only
+inet_protocols = ipv6

postfix/main.cf.Debian.buster

@@ -42,4 +42,5 @@ mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
 mailbox_size_limit = 0
 recipient_delimiter = +
 inet_interfaces = all
-inet_protocols = all
+# IPv6 only
+inet_protocols = ipv6

postfix/main.cf.ticket.in.ffho.net

@@ -39,4 +39,6 @@ mailbox_command = procmail -a "$EXTENSION"
 mailbox_size_limit = 0
 recipient_delimiter = +
 inet_interfaces = all
+# IPv6 only
+inet_protocols = ipv6
 compatibility_level = 2