diff --git a/setup/network-checks.sh b/setup/network-checks.sh
new file mode 100644
index 00000000..7e091165
--- /dev/null
+++ b/setup/network-checks.sh
@@ -0,0 +1,53 @@
+# Stop if the PRIMARY_HOSTNAME is listed in the Spamhaus Domain Block List.
+# The user might have chosen a name that was previously in use by a spammer
+# and will not be able to reliably send mail. Do this after any automatic
+# choices made above.
+if host $PRIMARY_HOSTNAME.dbl.spamhaus.org > /dev/null; then
+	echo
+	echo "The hostname you chose '$PRIMARY_HOSTNAME' is listed in the"
+	echo "Spamhaus Domain Block List. See http://www.spamhaus.org/dbl/"
+	echo "and http://www.spamhaus.org/query/domain/$PRIMARY_HOSTNAME."
+	echo
+	echo "You will not be able to send mail using this domain name, so"
+	echo "setup cannot continue."
+	echo
+	exit
+fi
+
+# Stop if the IPv4 address is listed in the ZEN Spamhouse Block List.
+# The user might have ended up on an IP address that was previously in use
+# by a spammer, or the user may be deploying on a residential network. We
+# will not be able to reliably send mail in these cases.
+REVERSED_IPV4=$(echo $PUBLIC_IP | sed "s/\([0-9]*\).\([0-9]*\).\([0-9]*\).\([0-9]*\)/\4.\3.\2.\1/")
+if host $REVERSED_IPV4.zen.spamhaus.org > /dev/null; then
+	echo
+	echo "The IP address $PUBLIC_IP is listed in the Spamhaus Block List."
+	echo "See http://www.spamhaus.org/query/ip/$PUBLIC_IP."
+	echo
+	echo "You will not be able to send mail using this machine, so setup"
+	echo "cannot continue."
+	echo
+	echo "Associate a different IP address with this machine if possible."
+	echo "Many residential network IP addresses are listed, so Mail-in-a-Box"
+	echo "typically cannot be used on a residential Internet connection."
+	echo
+	exit
+fi
+
+# Stop if we cannot make an outbound connection on port 25. Many residential
+# networks block outbound port 25 to prevent their network from sending spam.
+# See if we can reach one of Google's MTAs with a 5-second timeout.
+if ! nc -z -w5 aspmx.l.google.com 25; then
+	echo
+	echo "Outbound mail (port 25) seems to be blocked by your network."
+	echo
+	echo "You will not be able to send mail using this machine, so setup"
+	echo "cannot continue."
+	echo
+	echo "Many residential networks block port 25 to prevent hijacked"
+	echo "machines from being able to send spam. I just tried to connect"
+	echo "to Google's mail server on port 25 but the connection did not"
+	echo "succeed."
+	echo
+	exit
+fi
diff --git a/setup/start.sh b/setup/start.sh
index af4ac1ea..db05587d 100755
--- a/setup/start.sh
+++ b/setup/start.sh
@@ -177,6 +177,8 @@ if [ "$PRIMARY_HOSTNAME" = "auto-easy" ]; then
 	echo "Primary Hostname: $PRIMARY_HOSTNAME"
 fi
 
+# Run some network checks to make sure setup on this machine makes sense.
+. setup/network-checks.sh
 
 # Create the user named "user-data" and store all persistent user
 # data (mailboxes, etc.) in that user's home directory.