diff --git a/matrix_webhook/handler.py b/matrix_webhook/handler.py
index 1f1ac5f..3460810 100644
--- a/matrix_webhook/handler.py
+++ b/matrix_webhook/handler.py
@@ -76,6 +76,11 @@ async def matrix_webhook(request):
     else:
         formatted_body = markdown(str(data["body"]), extensions=["extra"])
 
+    # try to join room first -> non none response means error
+    resp = await utils.join_room(data["room_id"])
+    if resp is not None:
+        return resp
+
     content = {
         "msgtype": "m.text",
         "body": data["body"],
diff --git a/matrix_webhook/utils.py b/matrix_webhook/utils.py
index 2b4d74d..4c7917a 100644
--- a/matrix_webhook/utils.py
+++ b/matrix_webhook/utils.py
@@ -6,7 +6,7 @@ from http import HTTPStatus
 from aiohttp import web
 from nio import AsyncClient
 from nio.exceptions import LocalProtocolError
-from nio.responses import RoomSendError
+from nio.responses import RoomSendError, JoinError
 
 from . import conf
 
@@ -15,6 +15,13 @@ LOGGER = logging.getLogger("matrix_webhook.utils")
 CLIENT = AsyncClient(conf.MATRIX_URL, conf.MATRIX_ID)
 
 
+def error_map(resp):
+    """Map response errors to HTTP status."""
+    if resp.status_code == "M_UNKNOWN":
+        return resp.transport_response.status
+    return ERROR_MAP[resp.status_code]
+
+
 def create_json_response(status, ret):
     """Create a JSON response."""
     LOGGER.debug(f"Creating json response: {status=}, {ret=}")
@@ -22,6 +29,27 @@ def create_json_response(status, ret):
     return web.json_response(response_data, status=status)
 
 
+async def join_room(room_id):
+    """Try to join the room."""
+    LOGGER.debug(f"Join room {room_id=}")
+
+    for _ in range(10):
+        try:
+            resp = await CLIENT.join(room_id)
+            if isinstance(resp, JoinError):
+                if resp.status_code == "M_UNKNOWN_TOKEN":
+                    LOGGER.warning("Reconnecting")
+                    await CLIENT.login(conf.MATRIX_PW)
+                else:
+                    return create_json_response(error_map(resp), resp.message)
+            else:
+                return None
+        except LocalProtocolError as e:
+            LOGGER.error(f"Send error: {e}")
+        LOGGER.warning("Trying again")
+    return create_json_response(HTTPStatus.GATEWAY_TIMEOUT, "Homeserver not responding")
+
+
 async def send_room_message(room_id, content):
     """Send a message to a room."""
     LOGGER.debug(f"Sending room message in {room_id=}: {content=}")
@@ -36,9 +64,7 @@ async def send_room_message(room_id, content):
                     LOGGER.warning("Reconnecting")
                     await CLIENT.login(conf.MATRIX_PW)
                 else:
-                    return create_json_response(
-                        ERROR_MAP[resp.status_code], resp.message
-                    )
+                    return create_json_response(error_map(resp), resp.message)
             else:
                 return create_json_response(HTTPStatus.OK, "OK")
         except LocalProtocolError as e:
diff --git a/tests/tests.py b/tests/tests.py
index aa5991c..7d44d7e 100644
--- a/tests/tests.py
+++ b/tests/tests.py
@@ -33,11 +33,11 @@ class BotTest(unittest.IsolatedAsyncioTestCase):
         # this won't be a 403 from synapse, but a LocalProtocolError from matrix_webhook
         self.assertEqual(
             bot_req({"body": 3}, KEY, "wrong_room"),
-            {"status": 403, "ret": "Unknown room"},
+            {"status": 400, "ret": "wrong_room was not legal room ID or room alias"},
         )
         self.assertEqual(
             bot_req({"body": 3}, KEY, "wrong_room", key_as_param=True),
-            {"status": 403, "ret": "Unknown room"},
+            {"status": 400, "ret": "wrong_room was not legal room ID or room alias"},
         )
 
     async def test_message(self):