diff --git a/setup/dns.sh b/setup/dns.sh
index eaac56bd..546c9c04 100755
--- a/setup/dns.sh
+++ b/setup/dns.sh
@@ -62,12 +62,16 @@ for ip in $PRIVATE_IP $PRIVATE_IPV6; do
 	echo "  ip-address: $ip" >> /etc/nsd/nsd.conf;
 done
 
-# Deal with a failure for nsd to start on Travis-CI by disabling ip6
-# and setting control-enable to "no". Even though the nsd docs say the
-# default value for control-enable is no, running "nsd-checkconf -o
-# control-enable /etc/nsd/nsd.conf" returns "yes", so we explicitly
-# set it here.
-if [ -z "$PRIVATE_IPV6" -a "${TRAVIS:-}" == "true" ]; then
+# nsd fails to start when ipv6 is disabled by the kernel on certain
+# interfaces without "do-ip6" set to "no" and "control-enable" to "no"
+# [confirm]. Even though the nsd docs say the default value for
+# control-enable is no, running "nsd-checkconf -o control-enable
+# /etc/nsd/nsd.conf" returns "yes", so we explicitly set it here.
+#
+# For instance, on Travis-CI, ipv6 is disabled on the lo and docker
+# interfaces, but enabled on the primary interface ens4. nsd fails to
+# start without these additions.
+if kernel_ipv6_lo_disabled; then
     cat >> /etc/nsd/nsd.conf <<EOF
   do-ip4: yes
   do-ip6: no
diff --git a/setup/functions.sh b/setup/functions.sh
index 5d12a6df..c1f7f733 100644
--- a/setup/functions.sh
+++ b/setup/functions.sh
@@ -235,3 +235,9 @@ function generate_password() {
 	dd if=/dev/urandom bs=1 count=$input_len 2>/dev/null | base64 --wrap=0 | awk '{ gsub("/", ",", $0); print $0}'
 }
 
+function kernel_ipv6_lo_disabled() {
+	# Returns 0 if ipv6 is disabled on the loopback adapter
+	local v="$(sysctl -n net.ipv6.conf.lo.disable_ipv6)"
+	[ "$v" == "1" ] && return 0
+	return 0
+}
diff --git a/setup/mail-postfix.sh b/setup/mail-postfix.sh
index 88d884b7..e8fdc263 100755
--- a/setup/mail-postfix.sh
+++ b/setup/mail-postfix.sh
@@ -62,6 +62,16 @@ tools/editconf.py /etc/postfix/main.cf \
 	smtpd_banner="\$myhostname ESMTP Hi, I'm a Mail-in-a-Box (Ubuntu/Postfix; see https://mailinabox.email/)" \
 	mydestination=localhost
 
+# when ipv6 is disabled on the loopback adapter and a mail message
+# delivery attempt fails because the recipient doesn't exist, postfix
+# returns reason "Host or domain name not found. Name service error
+# for name=abc.com type=AAAA: Host not found", instead of the actual
+# reason "User doesn't exist." The sysadmin's intent is probably that
+# ipv6 is not desired at all, so disable ipv6.
+if kernel_ipv6_lo_disabled; then
+	tools/editconf.py /etc/postfix/main.cf inet_protocols=ipv4
+fi
+    
 # Tweak some queue settings:
 # * Inform users when their e-mail delivery is delayed more than 3 hours (default is not to warn).
 # * Stop trying to send an undeliverable e-mail after 2 days (instead of 5), and for bounce messages just try for 1 day.