diff --git a/install_postfix_advanced.sh b/install_postfix_advanced.sh
index 46ec482..699ca52 100755
--- a/install_postfix_advanced.sh
+++ b/install_postfix_advanced.sh
@@ -1219,7 +1219,26 @@ else
 fi
 
 echononl "   Create file \"transport\""
-echo > /etc/postfix/transport
+cat <<EOF > /etc/postfix/transport
+# - Force sending through IPv4
+# -
+# - Take care your master.cf file (/etc/postfix/master.cf) contains:
+# -
+# -    smtp-ipv4-only unix  -       -       n        -       -       smtp
+# -       -o inet_protocols=ipv4
+# -
+#example.com   smtp-ipv4-only:
+
+# - Force sending through IPv6
+# -
+# - Take care your master.cf file (/etc/postfix/master.cf) contains:
+# -
+# -    smtp-ipv6-only unix  -       -       n        -       -       smtp
+# -       -o inet_protocols=ipv6
+# -
+#example.net   smtp-ipv6-only:
+
+EOF
 postmap btree:/etc/postfix/transport
 if [[ $? -eq 0 ]] ; then
    echo_ok
@@ -1425,6 +1444,17 @@ EOF
 
 	done < "${postfix_master_cf}.$backup_date"
 
+   # - Add transport definitions for only sending over IPv4/IPv6
+   # -
+   cat <<EOF >> $postfix_master_cf
+
+smtp-ipv4-only      unix  -       -       n       -       -       smtp
+   -o inet_protocols=ipv4
+
+smtp-ipv6-only      unix  -       -       n       -       -       smtp
+   -o inet_protocols=ipv6
+EOF
+
 	echo_done
 	warn "Please check file \"$postfix_master_cf\" !"
 else