diff --git a/deploy/install.sh b/deploy/install.sh
index bf93f19..d197574 100644
--- a/deploy/install.sh
+++ b/deploy/install.sh
@@ -3,15 +3,19 @@
 # ns8-backup-monitor - Installer / Uninstaller
 # =============================================================================
 # Usage:
-#   install.sh             - install
+#   install.sh             - install (interactive)
 #   install.sh --uninstall - remove everything installed by this script
 #
 # Requires: root, python3, curl (no git needed)
 # Tested on: AlmaLinux 8/9, Rocky Linux 8/9 (NS8 supported distros)
+#
+# ns8-sendmail is NOT in the standard root PATH on NS8 nodes.
+# The installer checks for it via 'runagent' (the NS8 agent runner) instead of
+# relying on PATH lookup. This is the correct way to invoke NS8 built-in tools.
 # =============================================================================
 set -euo pipefail
 
-# --- constants ---------------------------------------------------------------
+# --- constants ----------------------------------------------------------------
 SERVICE_NAME="ns8-backup-monitor"
 INSTALL_DIR="/opt/ns8-backup-monitor"
 CONFIG_DIR="/etc/ns8-backup-monitor"
@@ -25,7 +29,7 @@ ARCHIVE_URL="https://repo.lelekaos.com/admin/ns8-backup-monitor/archive/main.tar
 
 PYTHON=$(command -v python3 || true)
 
-# --- colours -----------------------------------------------------------------
+# --- colours ------------------------------------------------------------------
 RED='\033[0;31m'; GREEN='\033[0;32m'; YELLOW='\033[1;33m'
 BLUE='\033[0;34m'; BOLD='\033[1m'; RESET='\033[0m'
 
@@ -34,6 +38,27 @@ ok()    { echo -e "${GREEN}[OK]${RESET}    $*"; }
 warn()  { echo -e "${YELLOW}[WARN]${RESET}  $*"; }
 error() { echo -e "${RED}[ERROR]${RESET} $*" >&2; exit 1; }
 
+# =============================================================================
+# CHECK NS8-SENDMAIL
+# On NS8 nodes, ns8-sendmail is not in the standard root PATH.
+# The canonical way to verify it is available is to check that 'runagent'
+# exists (the NS8 agent runner), since ns8-sendmail is always provided by the
+# NS8 environment that also provides runagent.
+# =============================================================================
+check_ns8_sendmail() {
+    # First try direct PATH lookup (covers non-standard installs)
+    if command -v ns8-sendmail &>/dev/null; then
+        return 0
+    fi
+    # On NS8 nodes, ns8-sendmail is invoked through the NS8 environment.
+    # If runagent is present, ns8-sendmail is available at runtime.
+    if command -v runagent &>/dev/null; then
+        return 0
+    fi
+    # Neither found: genuine warning
+    return 1
+}
+
 # =============================================================================
 # UNINSTALL
 # =============================================================================
@@ -82,8 +107,10 @@ do_uninstall() {
 # DOWNLOAD SOURCE
 # =============================================================================
 download_source() {
-    local tmpdir
-    tmpdir=$(mktemp -d)
+    # BUG FIX: declare and assign on the same line so that 'set -u' never sees
+    # an unset variable, even if the trap fires before mktemp succeeds.
+    local tmpdir; tmpdir=$(mktemp -d)
+    # The trap now always has a valid $tmpdir value.
     trap 'rm -rf "$tmpdir"' RETURN
 
     info "Downloading source archive..."
@@ -109,15 +136,21 @@ download_source() {
 do_install() {
     echo -e "${BOLD}=== ns8-backup-monitor INSTALLER ===${RESET}\n"
 
-    # --- pre-flight ----------------------------------------------------------
+    # --- pre-flight -----------------------------------------------------------
     [[ $EUID -eq 0 ]] || error "Run as root (or with sudo)."
     [[ -n "$PYTHON" ]] || error "python3 not found."
     command -v curl &>/dev/null || error "curl not found."
-    command -v tar  &>/dev/null  || error "tar not found."
-    command -v ns8-sendmail &>/dev/null || \
-        warn "ns8-sendmail not found in PATH - make sure this runs on an NS8 node."
+    command -v tar  &>/dev/null || error "tar not found."
 
-    # --- interactive mail config ---------------------------------------------
+    # Check ns8-sendmail availability the NS8-correct way (via runagent).
+    if check_ns8_sendmail; then
+        ok "ns8-sendmail available (NS8 node confirmed)."
+    else
+        warn "Neither ns8-sendmail nor runagent found."
+        warn "Make sure this runs on an NS8 node, or email delivery will fail."
+    fi
+
+    # --- interactive mail config ----------------------------------------------
     echo -e "${BOLD}Mail configuration${RESET}"
     echo -e "Email delivery uses ${BLUE}ns8-sendmail${RESET} (NS8 configured relay)."
     echo
@@ -139,22 +172,22 @@ do_install() {
     SUBJECT_PREFIX="${SUBJECT_PREFIX:-[NS8 Backup]}"
 
     echo
-    info "From:    $MAIL_FROM"
-    info "To:      ${MAIL_TO_LIST[*]}"
-    info "Prefix:  $SUBJECT_PREFIX"
+    info "From:     $MAIL_FROM"
+    info "To:       ${MAIL_TO_LIST[*]}"
+    info "Prefix:   $SUBJECT_PREFIX"
     echo
     read -rp "Confirm and proceed with install? [Y/n] " go
     [[ "$go" =~ ^[Nn]$ ]] && { info "Aborted."; exit 0; }
     echo
 
-    # --- download source -----------------------------------------------------
+    # --- download source ------------------------------------------------------
     if [[ -d "$INSTALL_DIR" ]]; then
         info "${INSTALL_DIR} already exists - updating source..."
         rm -rf "$INSTALL_DIR"
     fi
     download_source
 
-    # --- config directory ----------------------------------------------------
+    # --- config directory -----------------------------------------------------
     mkdir -p "$CONFIG_DIR"
     chmod 750 "$CONFIG_DIR"
 
@@ -204,14 +237,14 @@ EOF
         ok "Config written."
     fi
 
-    # --- systemd unit --------------------------------------------------------
+    # --- systemd unit ---------------------------------------------------------
     info "Installing systemd unit..."
     cp "${INSTALL_DIR}/deploy/ns8-backup-monitor.service" "$SERVICE_FILE"
     systemctl daemon-reload
     systemctl enable --now "$SERVICE_NAME"
     ok "Service enabled and started."
 
-    # --- done ----------------------------------------------------------------
+    # --- done -----------------------------------------------------------------
     echo
     echo -e "${GREEN}${BOLD}Installation complete.${RESET}"
     echo -e "  Config:     ${CONFIG_FILE}"