diff --git a/matrix/encryption.py b/matrix/encryption.py
index cd3ffa7..292689d 100644
--- a/matrix/encryption.py
+++ b/matrix/encryption.py
@@ -59,6 +59,10 @@ class ParseError(Exception):
     pass
 
 
+class OlmTrustError(Exception):
+    pass
+
+
 class WeechatArgParse(argparse.ArgumentParser):
     def print_usage(self, file):
         pass
@@ -470,6 +474,7 @@ class Olm():
         self.session_path = session_path
         self.database = database
         self.device_keys = {}
+        self.shared_sessions = []
 
         if not database:
             db_file = "{}_{}.db".format(user, device_id)
@@ -615,7 +620,12 @@ class Olm():
 
         if room_id not in self.outbound_group_sessions:
             self.create_outbound_group_session(room_id)
+
+        if self.outbound_group_sessions[room_id].id not in self.shared_sessions:
             to_device_dict = self.share_group_session(room_id, own_id, users)
+            self.shared_sessions.append(
+                self.outbound_group_sessions[room_id].id
+            )
 
         session = self.outbound_group_sessions[room_id]
 
@@ -681,6 +691,9 @@ class Olm():
                 if not self.sessions[user][key.device_id]:
                     continue
 
+                if not self.trust_db.check(key):
+                    raise OlmTrustError
+
                 device_payload_dict = payload_dict.copy()
                 # TODO sort the sessions
                 session = self.sessions[user][key.device_id][0]
diff --git a/matrix/server.py b/matrix/server.py
index b84bc0a..98c6c69 100644
--- a/matrix/server.py
+++ b/matrix/server.py
@@ -44,7 +44,12 @@ from matrix.api import (
     MatrixKeyClaimMessage
 )
 
-from matrix.encryption import Olm, EncryptionError, encrypt_enabled
+from matrix.encryption import (
+    Olm,
+    EncryptionError,
+    OlmTrustError,
+    encrypt_enabled
+)
 
 try:
     FileNotFoundError
@@ -518,26 +523,34 @@ class MatrixServer:
 
         W.prnt("", "matrix: Encrypting message")
 
-        payload_dict, to_device_dict = self.olm.group_encrypt(
-            room_id,
-            plaintext_dict,
-            self.user_id,
-            room.users.keys()
-        )
+        try:
+            payload_dict, to_device_dict = self.olm.group_encrypt(
+                room_id,
+                plaintext_dict,
+                self.user_id,
+                room.users.keys()
+            )
+
+            if to_device_dict:
+                W.prnt("", "matrix: Megolm session missing for room.")
+                message = MatrixToDeviceMessage(self.client, to_device_dict)
+                self.send_queue.append(message)
+
+            message = MatrixEncryptedMessage(
+                self.client,
+                room_id,
+                formatted_data,
+                payload_dict
+            )
 
-        if to_device_dict:
-            W.prnt("", "matrix: Megolm session missing for room.")
-            message = MatrixToDeviceMessage(self.client, to_device_dict)
             self.send_queue.append(message)
 
-        message = MatrixEncryptedMessage(
-            self.client,
-            room_id,
-            formatted_data,
-            payload_dict
-        )
-
-        self.send_queue.append(message)
+        except OlmTrustError:
+            m = ("{prefix}matrix: Untrusted devices found in room, "
+                 "verification is needed before sending a message").format(
+                    prefix=W.prefix("error"))
+            W.prnt(self.server_buffer, m)
+            return
 
     @encrypt_enabled
     def upload_keys(self, device_keys=False, one_time_keys=False):