diff --git a/.gitignore b/.gitignore
index 0bb3ba4..0554d6e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -7,3 +7,6 @@ repositories/
 __pycache__/
 *.pyc
 flask_session/
+
+# Runtime lock files
+config/*.lock
diff --git a/simple_org_chart/app_main.py b/simple_org_chart/app_main.py
index a54b0da..3bc002b 100644
--- a/simple_org_chart/app_main.py
+++ b/simple_org_chart/app_main.py
@@ -1200,7 +1200,10 @@ def import_settings():
         elif key in email_keys:
             email_data[key] = value
 
-    settings_ok = save_settings(settings_data)
+    # Only save settings if at least one valid settings key was provided
+    settings_ok = True
+    if settings_data:
+        settings_ok = save_settings(settings_data)
 
     email_ok = True
     if email_data:
diff --git a/simple_org_chart/email_config.py b/simple_org_chart/email_config.py
index 57ef6aa..b4019ab 100644
--- a/simple_org_chart/email_config.py
+++ b/simple_org_chart/email_config.py
@@ -31,6 +31,12 @@
 # Keys that belong to the email configuration
 _EMAIL_KEYS = set(DEFAULT_EMAIL_CONFIG.keys())
 
+
+def _filter_email_keys(source: Dict[str, Any]) -> Dict[str, Any]:
+    """Return a new dict containing only keys that belong to the email config."""
+    return {key: source[key] for key in _EMAIL_KEYS if key in source}
+
+
 def get_smtp_config() -> Dict[str, Any]:
     """Load SMTP configuration from environment variables."""
     # Get encryption setting (TLS, SSL, or None)
@@ -61,14 +67,16 @@ def load_email_config() -> Dict[str, Any]:
         try:
             with SETTINGS_FILE.open("r", encoding="utf-8") as handle:
                 stored = json.load(handle)
-                merged = DEFAULT_EMAIL_CONFIG.copy()
-                for key in _EMAIL_KEYS:
-                    if key in stored:
-                        merged[key] = stored[key]
-                return merged
         except Exception as error:
             logger.error("Error loading email config: %s", error)
-    
+        else:
+            if not isinstance(stored, dict):
+                logger.warning("app_settings.json does not contain a JSON object; using email defaults")
+                return DEFAULT_EMAIL_CONFIG.copy()
+            merged = DEFAULT_EMAIL_CONFIG.copy()
+            merged.update(_filter_email_keys(stored))
+            return merged
+
     return DEFAULT_EMAIL_CONFIG.copy()
 
 
@@ -76,9 +84,9 @@ def save_email_config(config: Dict[str, Any]) -> bool:
     """Save email configuration into app_settings.json."""
     SETTINGS_FILE.parent.mkdir(parents=True, exist_ok=True)
 
-    # Merge with defaults
+    # Restrict to known email keys only — never let caller overwrite unrelated settings
     persisted = DEFAULT_EMAIL_CONFIG.copy()
-    persisted.update(config)
+    persisted.update(_filter_email_keys(config))
 
     if 'lastSent' not in config:
         # lastSent will be preserved from the existing file inside the lock below
diff --git a/simple_org_chart/settings.py b/simple_org_chart/settings.py
index 67763d1..2913637 100644
--- a/simple_org_chart/settings.py
+++ b/simple_org_chart/settings.py
@@ -13,10 +13,83 @@
 
 from .config import SETTINGS_FILE
 
-# Shared in-process lock protecting all reads/writes to SETTINGS_FILE.
+
+class _InterProcessSettingsFileLock:
+    """Combined thread + process lock for protecting SETTINGS_FILE.
+
+    Serialises concurrent access across both threads within a single worker
+    and across multiple Gunicorn worker processes by combining a
+    ``threading.Lock`` with an ``fcntl.flock`` advisory file lock.  On
+    platforms where ``fcntl`` is unavailable (e.g. Windows) the class
+    falls back gracefully to thread-only locking.
+    """
+
+    def __init__(self, lock_file_path: str) -> None:
+        self._lock_file_path = lock_file_path
+        self._thread_lock = threading.Lock()
+        self._fd: int | None = None
+
+    def acquire(self, blocking: bool = True) -> bool:
+        acquired = self._thread_lock.acquire(blocking)
+        if not acquired:
+            return False
+        fd = None
+        try:
+            import fcntl
+            fd = os.open(self._lock_file_path, os.O_CREAT | os.O_RDWR, 0o600)
+            flags = fcntl.LOCK_EX | (0 if blocking else fcntl.LOCK_NB)
+            fcntl.flock(fd, flags)
+            self._fd = fd
+        except ImportError:
+            # fcntl not available (Windows); thread lock is sufficient.
+            if fd is not None:
+                os.close(fd)
+        except BlockingIOError:
+            if fd is not None:
+                os.close(fd)
+            self._thread_lock.release()
+            return False
+        except Exception:
+            if fd is not None:
+                os.close(fd)
+            self._thread_lock.release()
+            raise
+        return True
+
+    def release(self) -> None:
+        try:
+            if self._fd is not None:
+                try:
+                    import fcntl
+                    fcntl.flock(self._fd, fcntl.LOCK_UN)
+                except (ImportError, OSError):
+                    pass
+                finally:
+                    try:
+                        os.close(self._fd)
+                    except OSError:
+                        pass
+                    self._fd = None
+        finally:
+            self._thread_lock.release()
+
+    def __enter__(self) -> "_InterProcessSettingsFileLock":
+        self.acquire()
+        return self
+
+    def __exit__(self, exc_type: Any, exc_val: Any, exc_tb: Any) -> None:
+        self.release()
+
+
+# Shared inter-process lock protecting all reads/writes to SETTINGS_FILE.
 # Both settings.py and email_config.py import this lock so that concurrent
-# writes from different code paths (scheduler, HTTP handlers) are serialised.
-_settings_file_lock = threading.Lock()
+# writes from different code paths (scheduler, HTTP handlers, workers) are
+# serialised across all Gunicorn worker processes.
+_settings_lock_file = os.path.join(
+    os.fspath(SETTINGS_FILE.parent),
+    f"{SETTINGS_FILE.name}.lock",
+)
+_settings_file_lock = _InterProcessSettingsFileLock(_settings_lock_file)
 
 logger = logging.getLogger(__name__)