diff --git a/deploy_vps.sh b/deploy_vps.sh
index 147a715..ee84edf 100755
--- a/deploy_vps.sh
+++ b/deploy_vps.sh
@@ -46,23 +46,30 @@ set -euo pipefail
 sudo -v
 
 if ! id "$APP_USER" >/dev/null 2>&1; then
-  sudo useradd --system --home "$APP_DIR" --shell /usr/sbin/nologin "$APP_USER"
+  if command -v useradd >/dev/null 2>&1; then
+    sudo useradd --system --home "$APP_DIR" --shell /usr/sbin/nologin "$APP_USER"
+  else
+    APP_USER="$(id -un)"
+    echo "useradd not found on remote host; using existing SSH user '$APP_USER' for the service." >&2
+  fi
 fi
+APP_GROUP="$(id -gn "$APP_USER")"
 
 sudo mkdir -p "$APP_DIR" /var/lib/mailcat /var/log/mailcat
 sudo tar -xzf /tmp/mailcat-deploy.tar.gz -C "$APP_DIR"
-sudo chown -R "$APP_USER:$APP_USER" "$APP_DIR" /var/lib/mailcat /var/log/mailcat
+sudo chown -R "$APP_USER:$APP_GROUP" "$APP_DIR" /var/lib/mailcat /var/log/mailcat
 sudo chmod 750 "$APP_DIR"
 
 if [ ! -f "$APP_DIR/config.json" ]; then
   echo "Missing $APP_DIR/config.json on remote host. Create it with mode 600 before starting the service." >&2
 fi
 sudo chmod 600 "$APP_DIR/config.json" 2>/dev/null || true
-sudo chown "$APP_USER:$APP_USER" "$APP_DIR/config.json" 2>/dev/null || true
+sudo chown "$APP_USER:$APP_GROUP" "$APP_DIR/config.json" 2>/dev/null || true
 
 sudo install -m 0644 "$APP_DIR/systemd/mailcat-sort.service" "/etc/systemd/system/$SERVICE_NAME.service"
 sudo sed -i \
   -e "s#__APP_USER__#$APP_USER#g" \
+  -e "s#__APP_GROUP__#$APP_GROUP#g" \
   -e "s#__APP_DIR__#$APP_DIR#g" \
   -e "s#__ACCOUNT__#$ACCOUNT#g" \
   "/etc/systemd/system/$SERVICE_NAME.service"
diff --git a/restart_prompt.md b/restart_prompt.md
index 590fe83..4f4d60f 100644
--- a/restart_prompt.md
+++ b/restart_prompt.md
@@ -149,6 +149,7 @@ Remote/NAS deployment files:
 - `deploy_vps.sh` is host-neutral despite its historical filename. It packages the repo excluding `.git`, `config.json`, logs, runtime JSON state/log files, bytecode, and local mailbox data; uploads to the SSH host in `REMOTE_HOST`; installs under `/opt/mailcat` by default; creates/uses a non-root system user named `mailcat` by default; installs a systemd service; enables but does not start the service.
 - The deploy script requires `REMOTE_HOST` and `REMOTE_USER`. The old `VPS_HOST` and `VPS_USER` names still work as aliases. Optional overrides: `APP_USER`, `APP_DIR`, `SERVICE_NAME`, and `ACCOUNT`.
 - The deploy script uploads a temporary remote shell script and runs it with `ssh -tt`, so remote `sudo` can prompt for a password on NAS systems that require a terminal. The remote script starts with `sudo -v`.
+- If the remote NAS does not have `useradd`, the deploy script falls back to the existing SSH user and that user's primary group for service ownership.
 - `systemd/mailcat-sort.service` is a template consumed by `deploy_vps.sh`; after placeholder replacement it runs `sort_mail_daemon.py --account backup --folder INBOX` and stores state/logs under `/var/lib/mailcat` and `/var/log/mailcat`.
 - `config.json` remains ignored and must be created manually on the remote host with mode `600` before starting the service.
 
diff --git a/systemd/mailcat-sort.service b/systemd/mailcat-sort.service
index 37cb985..7b1bf6d 100644
--- a/systemd/mailcat-sort.service
+++ b/systemd/mailcat-sort.service
@@ -6,7 +6,7 @@ Wants=network-online.target
 [Service]
 Type=simple
 User=__APP_USER__
-Group=__APP_USER__
+Group=__APP_GROUP__
 WorkingDirectory=__APP_DIR__
 ExecStart=/usr/bin/python3 __APP_DIR__/sort_mail_daemon.py --account __ACCOUNT__ --folder INBOX --state-file /var/lib/mailcat/sort_mail_daemon_state.json --log-file /var/log/mailcat/sort_mail_daemon.log
 Restart=always