diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6d40cb5e3..d4d430389 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -13,6 +13,8 @@ CHANGELOG
 ### Configuration
 
 ### Core
+- `intelmq.lib.message`: For invalid message keys, add a hint on the failure to the exception: not allowed by configuration or not matching regular expression (PR#2398 by Sebastian Wagner).
+- `intelmq.lib.exceptions.InvalidKey`: Add optional parameter `additional_text` (PR#2398 by Sebastian Wagner).
 
 ### Development
 
diff --git a/intelmq/lib/exceptions.py b/intelmq/lib/exceptions.py
index fa9df6d75..110e6d70c 100644
--- a/intelmq/lib/exceptions.py
+++ b/intelmq/lib/exceptions.py
@@ -88,8 +88,8 @@ def __init__(self, key: str, value: str, reason: Any = None, object: bytes = Non
 
 class InvalidKey(IntelMQHarmonizationException, KeyError):
 
-    def __init__(self, key: str):
-        message = "invalid key %s" % repr(key)
+    def __init__(self, key: str, additional_text: Optional[str] = None):
+        message = f"invalid key {key!r} {additional_text or ''}".strip()  # remove trailing whitespace if additional_text is not given
         super().__init__(message)
 
 
diff --git a/intelmq/lib/message.py b/intelmq/lib/message.py
index 2b116bb6b..e99e22731 100644
--- a/intelmq/lib/message.py
+++ b/intelmq/lib/message.py
@@ -13,7 +13,7 @@
 import re
 import warnings
 from collections import defaultdict
-from typing import Any, Dict, Iterable, Optional, Sequence, Union
+from typing import Any, Dict, Iterable, Optional, Sequence, Union, Tuple
 from pkg_resources import resource_filename
 
 import intelmq.lib.exceptions as exceptions
@@ -186,8 +186,9 @@ def is_valid(self, key: str, value: str, sanitize: bool = True) -> bool:
             intelmq.lib.exceptions.InvalidKey: if given key is invalid.
 
         """
-        if not self.__is_valid_key(key):
-            raise exceptions.InvalidKey(key)
+        key_validation = self.__is_valid_key(key)
+        if not key_validation[0]:
+            raise exceptions.InvalidKey(key, additional_text=key_validation[1])
 
         if value is None or value in ["", "-", "N/A"]:
             return False
@@ -243,8 +244,9 @@ def add(self, key: str, value: str, sanitize: bool = True,
                 del self[key]
             return
 
-        if not self.__is_valid_key(key):
-            raise exceptions.InvalidKey(key)
+        key_validation = self.__is_valid_key(key)
+        if not key_validation[0]:
+            raise exceptions.InvalidKey(key, additional_text=key_validation[1])
 
         try:
             if value in ignore:
@@ -330,16 +332,16 @@ def unserialize(message_string: str):
         message = json.loads(message_string)
         return message
 
-    def __is_valid_key(self, key: str):
+    def __is_valid_key(self, key: str) -> Tuple[bool, str]:
         try:
             class_name, subitem = self.__get_type_config(key)
         except KeyError:
-            return False
+            return False, 'This key is not allowed by the harmonization configuration'
         if key in self.harmonization_config or key == '__type':
-            return True
+            return True, None
         if subitem:
-            return HARMONIZATION_KEY_FORMAT.match(key)
-        return False
+            return HARMONIZATION_KEY_FORMAT.match(key), f'Does not match regular expression {HARMONIZATION_KEY_FORMAT.pattern}'
+        return False, 'This key is not allowed by the harmonization configuration'
 
     def __is_valid_value(self, key: str, value: str):
         if key == '__type':
@@ -569,7 +571,7 @@ def __init__(self, message: Union[dict, tuple] = (), auto: bool = False,
         if isinstance(message, Event):
             super().__init__({}, auto, harmonization)
             for key, value in message.items():
-                if self._Message__is_valid_key(key):
+                if self._Message__is_valid_key(key)[0]:
                     self.add(key, value, sanitize=False)
         else:
             super().__init__(message, auto, harmonization)
diff --git a/intelmq/tests/lib/test_exceptions.py b/intelmq/tests/lib/test_exceptions.py
index 1ad2d49bf..ee9c2a417 100755
--- a/intelmq/tests/lib/test_exceptions.py
+++ b/intelmq/tests/lib/test_exceptions.py
@@ -64,6 +64,13 @@ def test_MissingDependencyError(self):
         self.assertIn(repr(depname), exc)
         self.assertTrue(exc.endswith(" %s" % additional))
 
+    def test_invalid_key(self):
+        """
+        Check intelmq.lib.exceptions.InvalidKey
+        """
+        exc = excs.InvalidKey('test_key', additional_text='TEST').args[0]
+        self.assertTrue(exc.endswith(' TEST'))
+
 
 if __name__ == '__main__':  # pragma: no cover
     unittest.main()
diff --git a/intelmq/tests/lib/test_message.py b/intelmq/tests/lib/test_message.py
index 9f9c2ddb4..ac1e1cbcd 100644
--- a/intelmq/tests/lib/test_message.py
+++ b/intelmq/tests/lib/test_message.py
@@ -168,8 +168,10 @@ def test_invalid_type2(self):
     def test_report_invalid_key(self):
         """ Test if report raises InvalidKey for invalid key in add(). """
         report = self.new_report()
-        with self.assertRaises(exceptions.InvalidKey):
+        with self.assertRaises(exceptions.InvalidKey) as cm:
             report.add('invalid', 0)
+        self.assertIn('not allowed', cm.exception.args[0])
+
 
     def test_report_add_raw(self):
         """ Test if report can add raw value. """
@@ -764,10 +766,11 @@ def test_invalid_harm_key(self):
             message.Event(harmonization={'event': {'foo.bar.': {}}})
 
     def test_invalid_extra_key_name(self):
-        """ Test if error is raised if an extra field name is invalid. """
+        """ Test if error is raised when an extra field name is invalid and error message is included in exception. """
         event = message.Event(harmonization=HARM)
-        with self.assertRaises(exceptions.InvalidKey):
+        with self.assertRaises(exceptions.InvalidKey) as cm:
             event.add('extra.foo-', 'bar')
+        self.assertIn('Does not match regular expression', cm.exception.args[0])
 
 
 class TestReport(unittest.TestCase):