diff --git a/CHANGELOG.md b/CHANGELOG.md
index bf4e176e..9e47d9cb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,7 @@
 * Slow mode (#192):
   * new option in room configuration to set the slow mode delay (new prosody module mod_muc_slow_mode).
   * default delay is configurable in channel's chat rooms options.
+  * backend rejects messages when the slow mode is not respected.
 
 ### Minor changes and fixes
 
diff --git a/prosody-modules/mod_muc_slow_mode/mod_muc_slow_mode.lua b/prosody-modules/mod_muc_slow_mode/mod_muc_slow_mode.lua
index 7471a510..1a0001b4 100644
--- a/prosody-modules/mod_muc_slow_mode/mod_muc_slow_mode.lua
+++ b/prosody-modules/mod_muc_slow_mode/mod_muc_slow_mode.lua
@@ -9,10 +9,19 @@
 -- Implements: XEP-????: MUC Slow Mode (XEP to come).
 --
 -- Imports
+local st = require "util.stanza";
+local jid_bare = require "util.jid".bare;
+local gettime = require 'socket'.gettime;
 
 -- Plugin dependencies
 local mod_muc = module:depends "muc";
 
+local muc_util = module:require "muc/util";
+local valid_roles = muc_util.valid_roles;
+
+-- Namespaces
+local xmlns_muc = "http://jabber.org/protocol/muc";
+
 -- Getter/Setter
 local function get_slow_mode_delay(room)
 	return room._data.slow_mode_delay or 0;
@@ -60,3 +69,80 @@ module:hook("muc-config-submitted/muc#roomconfig_slow_mode_delay", function(even
 end);
 
 module:hook("muc-config-form", add_form_option, 100-4);
+
+-- handling groupchat messages
+function handle_groupchat(event)
+  local origin, stanza = event.origin, event.stanza;
+  local room = event.room;
+
+  local delay = get_slow_mode_delay(room) or 0;
+  if delay <= 0 then
+    -- no slow mode for this room
+    -- module:log("debug", "No slow mode for this room");
+    return;
+  end
+
+  -- Checking user's permissions (moderators are not subject to slow mode)
+	local actor = stanza.attr.from;
+	local actor_nick = room:get_occupant_jid(actor);
+  local actor_jid = jid_bare(actor);
+  -- Only checking role, not affiliation (slow mode only applies on users currently connected to the room)
+  local role = room:get_role(actor_nick);
+	if valid_roles[role or "none"] >= valid_roles.moderator then
+    -- user bypasses the slow mode.
+    -- module:log("debug", "User is moderator, bypassing slow mode");
+		return;
+	end
+
+  if not room.slow_mode_last_messages then
+    -- We store last message time for each users in room.slow_mode_last_messages:
+    -- * key: bare jid (without the nickname)
+    -- * value: last message timestamp
+    -- If room is cleared from memory, these data are lost. But should not be an issue.
+    -- For now, i don't clean slow_mode_last_messages, it should not use too much memory.
+    -- module:log("debug", "Initializing slow_mode_last_messages for the room.");
+    room.slow_mode_last_messages = {};
+  end
+
+  local now = gettime();
+  local previous = room.slow_mode_last_messages[actor_jid];
+  -- module:log(
+  --   "debug",
+  --   "Last message for user %s was at %s, now is %s, delay is %s, now - previous is %s",
+  --   actor_jid,
+  --   previous or 0,
+  --   now,
+  --   delay,
+  --   (now - (previous or 0))
+  -- );
+  if ((not previous) or (now - previous > delay)) then
+    -- module:log("debug", "Message accepted");
+    room.slow_mode_last_messages[actor_jid] = now;
+    return;
+  end
+
+  module:log("debug", "Bouncing message for user %s", actor_nick);
+  local reply = st.error_reply(
+    stanza,
+    -- error_type = 'wait' (see descriptions in RFC 6120 https://xmpp.org/rfcs/rfc6120.html#stanzas-error-syntax)
+    "wait",
+    -- error_condition = 'policy-violation' (see RFC 6120 Defined Error Conditions https://xmpp.org/rfcs/rfc6120.html#stanzas-error-conditions)
+    "policy-violation",
+    "You have exceeded the limit imposed by the slow mode in this room. You have to wait " .. delay .. " seconds between messages. Please try again later"
+  );
+
+  -- Note: following commented lines were inspired by mod_muc_limits, but it seems it is not required.
+  -- local body = stanza:get_child_text("body");
+  -- if body then
+  --   reply:up():tag("body"):text(body):up();
+  -- end
+  -- local x = stanza:get_child("x", xmlns_muc);
+  -- if x then
+  --   reply:add_child(st.clone(x));
+  -- end
+
+  origin.send(reply);
+  return true; -- stoping propagation
+end
+
+module:hook("muc-occupant-groupchat", handle_groupchat);