diff --git a/entrypoint.sh b/entrypoint.sh
index 475193c..4d31fd4 100755
--- a/entrypoint.sh
+++ b/entrypoint.sh
@@ -8,70 +8,86 @@ convertCase () {
     printf "%s" "$1" | tr "[:lower:]" "[:upper:]"
 }
 
-printf "\nVerifying environment variables... "
+if [ -f "/etc/postfix/main.cf.override" ]; then
+    # use provided configuration file
+    printf "\nAppending provided MAIN configuration... "
+    cat /etc/postfix/main.cf.override >> /etc/postfix/main.cf
+    \rm -f /tmp/main.cf.insert
+else
+    # process auto-setup
+    printf "\nVerifying environment variables... "
 
-# check for missing environment variable values
-if [ -z "$SMARTHOST" ]; then
-    printf "\nYou must specify the hostname or IP address of a smarthost where mail should be relayed.\n\n"
-fi
-if [ -z "$SMARTHOST_USERNAME" ]; then
-    printf "\nYou must provide a username for smarthost authentication.\n\n"
-fi
-if [ -z "$SMARTHOST_PASSWORD" ]; then
-    printf "\nYou must provide a password for smarthost authentication.\n\n"
-fi
+    # check for missing environment variable values
+    if [ -z "$SMARTHOST" ]; then
+        printf "\nYou must specify the hostname or IP address of a smarthost where mail should be relayed.\n\n"
+    fi
+    if [ -z "$SMARTHOST_USERNAME" ]; then
+        printf "\nYou must provide a username for smarthost authentication.\n\n"
+    fi
+    if [ -z "$SMARTHOST_PASSWORD" ]; then
+        printf "\nYou must provide a password for smarthost authentication.\n\n"
+    fi
 
-# set failsafes
-[ -z "$SMARTHOST_PORT" ] && SMARTHOST_PORT=587
-[ -z "$LOCAL_HOSTNAME" ] && LOCAL_HOSTNAME="smarthost"
-[ -z "$LOCAL_DOMAINNAME" ] && LOCAL_DOMAINNAME="${LOCAL_HOSTNAME#*.}"
+    # set failsafes
+    [ -z "$SMARTHOST_PORT" ] && SMARTHOST_PORT=587
+    [ -z "$LOCAL_HOSTNAME" ] && LOCAL_HOSTNAME="smarthost"
+    [ -z "$LOCAL_DOMAINNAME" ] && LOCAL_DOMAINNAME="${LOCAL_HOSTNAME#*.}"
 
+    printf "done\n"
+    printf "updating configuration files... "
+
+    # update main.cf
+    sed -i "s/{LOCAL_HOSTNAME}/${LOCAL_HOSTNAME}/" /tmp/main.cf.insert
+    sed -i "s/{LOCAL_DOMAINNAME}/${LOCAL_DOMAINNAME}/" /tmp/main.cf.insert
+    sed -i "s/{SMARTHOST}/${SMARTHOST}/" /tmp/main.cf.insert
+    sed -i "s/{SMARTHOST_PORT}/${SMARTHOST_PORT}/" /tmp/main.cf.insert
+    sed -i "s/{SMARTHOST_USERNAME}/${SMARTHOST_USERNAME}/" /tmp/main.cf.insert
+    sed -i "s/{SMARTHOST_PASSWORD}/${SMARTHOST_PASSWORD}/" /tmp/main.cf.insert
+
+    LOCAL_ENCRYPTION="$(convertCase "$LOCAL_ENCRYPTION")"
+    case "$LOCAL_ENCRYPTION" in
+        OPT*)
+            sed -i 's/{LOCAL_ENCRYPTION}/may/' /tmp/main.cf.insert
+            sed -i 's/#smtpd_/smtpd_/g' /tmp/main.cf.insert
+            ;;
+        TRUE)
+            sed -i 's/{LOCAL_ENCRYPTION}/encrypt/' /tmp/main.cf.insert
+            sed -i 's/#smtpd_/smtpd_/g' /tmp/main.cf.insert
+            ;;
+        *)
+            sed -i 's/{LOCAL_ENCRYPTION}//' /tmp/main.cf.insert
+            ;;
+    esac
+
+    SMARTHOST_ENCRYPTION="$(convertCase "$SMARTHOST_ENCRYPTION")"
+    case "$SMARTHOST_ENCRYPTION" in
+        OPT*)
+            sed -i 's/{SMARTHOST_ENCRYPTION}/may/' /tmp/main.cf.insert
+            ;;
+        TRUE)
+            sed -i 's/{SMARTHOST_ENCRYPTION}/secure/' /tmp/main.cf.insert
+            ;;
+        *)
+            sed -i 's/{SMARTHOST_ENCRYPTION}/none/' /tmp/main.cf.insert
+            ;;
+    esac
+
+    # append configuration and remove temp file
+    cat /tmp/main.cf.insert >> /etc/postfix/main.cf
+    \rm -f /tmp/main.cf.insert
+fi
 printf "done\n"
-printf "updating configuration files... "
-
-# update main.cf
-sed -i "s/{LOCAL_HOSTNAME}/${LOCAL_HOSTNAME}/" /tmp/main.cf.insert
-sed -i "s/{LOCAL_DOMAINNAME}/${LOCAL_DOMAINNAME}/" /tmp/main.cf.insert
-sed -i "s/{SMARTHOST}/${SMARTHOST}/" /tmp/main.cf.insert
-sed -i "s/{SMARTHOST_PORT}/${SMARTHOST_PORT}/" /tmp/main.cf.insert
-sed -i "s/{SMARTHOST_USERNAME}/${SMARTHOST_USERNAME}/" /tmp/main.cf.insert
-sed -i "s/{SMARTHOST_PASSWORD}/${SMARTHOST_PASSWORD}/" /tmp/main.cf.insert
-
-LOCAL_ENCRYPTION="$(convertCase "$LOCAL_ENCRYPTION")"
-case "$LOCAL_ENCRYPTION" in
-    OPT*)
-        sed -i 's/{LOCAL_ENCRYPTION}/may/' /tmp/main.cf.insert
-        sed -i 's/#smtpd_/smtpd_/g' /tmp/main.cf.insert
-        ;;
-    TRUE)
-        sed -i 's/{LOCAL_ENCRYPTION}/encrypt/' /tmp/main.cf.insert
-        sed -i 's/#smtpd_/smtpd_/g' /tmp/main.cf.insert
-        ;;
-    *)
-        sed -i 's/{LOCAL_ENCRYPTION}//' /tmp/main.cf.insert
-        ;;
-esac
-
-SMARTHOST_ENCRYPTION="$(convertCase "$SMARTHOST_ENCRYPTION")"
-case "$SMARTHOST_ENCRYPTION" in
-    OPT*)
-        sed -i 's/{SMARTHOST_ENCRYPTION}/may/' /tmp/main.cf.insert
-        ;;
-    TRUE)
-        sed -i 's/{SMARTHOST_ENCRYPTION}/secure/' /tmp/main.cf.insert
-        ;;
-    *)
-        sed -i 's/{SMARTHOST_ENCRYPTION}/none/' /tmp/main.cf.insert
-        ;;
-esac
-
-cat /tmp/main.cf.insert >> /etc/postfix/main.cf
-rm -f /tmp/main.cf.insert
-
-# update master.cf
-sed -i 's/#tlsproxy/tlsproxy/' /etc/postfix/master.cf
 
+if [ -f "/etc/postfix/master.cf.override" ]; then
+    # use provided configuration file
+    printf "\nUsing provided MASTER configuration... "
+    \cp --force /etc/postfix/master.cf.override /etc/postfix/master.cf
+else
+    # update master.cf
+    sed -i 's/#tlsproxy/tlsproxy/' /etc/postfix/master.cf
+fi
 printf "done\n"
+
 printf "container setup complete!\n"
 
 # run CMD passed to this container