diff --git a/Dockerfile b/Dockerfile
index c4c9111..7af5cc7 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -78,7 +78,7 @@ RUN set -x ; \
   DEBIAN_FRONTEND="noninteractive" apt-get -y update && \
   DEBIAN_FRONTEND="noninteractive" apt-get -y upgrade && \
   DEBIAN_FRONTEND="noninteractive" apt-get -y install \
-    wget ca-certificates gnupg \
+    wget ca-certificates gnupg iproute2 \
     $( cat /tmp/derived-deps ) \
     libclass-c3-xs-perl \
     texlive-plain-generic texlive-latex-recommended texlive-fonts-recommended \
@@ -110,7 +110,9 @@ WORKDIR /srv/ledgersmb
 # Configure outgoing mail to use host, other run time variable defaults
 
 ## MAIL
-ENV LSMB_MAIL_SMTPHOST 172.17.0.1
+# '__CONTAINER_GATEWAY__' is a magic value which will be substituted
+# with the actual gateway IP address
+ENV LSMB_MAIL_SMTPHOST __CONTAINER_GATEWAY__
 #ENV LSMB_MAIL_SMTPPORT 25
 #ENV LSMB_MAIL_SMTPSENDER_HOSTNAME (container hostname)
 #ENV LSMB_MAIL_SMTPTLS
diff --git a/start.sh b/start.sh
index 2f193c1..d1fecdc 100755
--- a/start.sh
+++ b/start.sh
@@ -67,7 +67,6 @@ if [[ -n "$LSMB_HAVE_DEPRECATED" ]]; then
     echo "!!! DEPRECATED \$SSMTP_* PARAMETERS WILL BE REMOVED in the 1.9 image!!!"
 fi
 
-
 if [[ ! -f ./local/conf/ledgersmb.yaml ]]; then
   cat <<EOF >./local/conf/ledgersmb.yaml
 paths:
@@ -105,6 +104,12 @@ EOF
 
   if [[ -n "$LSMB_MAIL_SMTPHOST" ]]
   then
+      if [[ "$LSMB_MAIL_SMTPHOST" == "__CONTAINER_GATEWAY__" ]]
+      then
+         LSMB_MAIL_SMTPHOST="$(ip route | awk '/default/ { print $3 }')"
+         export LSMB_MAIL_SMTPHOST
+      fi
+
       cat <<EOF >./local/conf/ledgersmb.000.yaml
 mail:
   transport: