diff --git a/lib/matrix_server/room_server.ex b/lib/matrix_server/room_server.ex
index 1c9daca..54dd12f 100644
--- a/lib/matrix_server/room_server.ex
+++ b/lib/matrix_server/room_server.ex
@@ -13,7 +13,17 @@ defmodule MatrixServer.RoomServer do
   import Ecto.Query
   import Ecto.Changeset
 
-  alias MatrixServer.{Repo, Room, Event, StateResolution, Account, JoinedRoom}
+  alias MatrixServer.{
+    Repo,
+    Room,
+    Event,
+    StateResolution,
+    Account,
+    JoinedRoom,
+    Device,
+    DeviceTransaction
+  }
+
   alias MatrixServer.StateResolution.Authorization
   alias MatrixServerWeb.Client.Request.{CreateRoom, Kick, Ban}
 
@@ -154,6 +164,15 @@ defmodule MatrixServer.RoomServer do
     GenServer.call(pid, {:set_visibility, account, visibility})
   end
 
+  @doc """
+  Send a message to this room.
+  """
+  @spec send_message(pid(), Account.t(), Device.t(), String.t(), map(), String.t()) ::
+          {:ok, String.t()} | {:error, atom()}
+  def send_message(pid, account, device, event_type, content, txn_id) do
+    GenServer.call(pid, {:send_message, account, device, event_type, content, txn_id})
+  end
+
   ### Implementation
 
   @impl true
@@ -248,8 +267,8 @@ defmodule MatrixServer.RoomServer do
   def handle_call({:invite, account, user_id}, _from, %{room: room, state_set: state_set} = state) do
     invite_event = Event.Invite.new(room, account, user_id)
 
-    case insert_single_event(room, state_set, invite_event) do
-      {:ok, {state_set, room}} -> {:reply, :ok, %{state | state_set: state_set, room: room}}
+    case Repo.transaction(insert_single_event(room, state_set, invite_event)) do
+      {:ok, {state_set, room, _}} -> {:reply, :ok, %{state | state_set: state_set, room: room}}
       {:error, reason} -> {:reply, {:error, reason}, state}
     end
   end
@@ -261,8 +280,8 @@ defmodule MatrixServer.RoomServer do
       ) do
     join_event = Event.Join.new(room, account)
 
-    case insert_single_event(room, state_set, join_event) do
-      {:ok, {state_set, room}} ->
+    case Repo.transaction(insert_single_event(room, state_set, join_event)) do
+      {:ok, {state_set, room, _}} ->
         {:reply, {:ok, room_id}, %{state | state_set: state_set, room: room}}
 
       {:error, reason} ->
@@ -273,8 +292,8 @@ defmodule MatrixServer.RoomServer do
   def handle_call({:leave, account}, _from, %{room: room, state_set: state_set} = state) do
     leave_event = Event.Leave.new(room, account)
 
-    case insert_single_event(room, state_set, leave_event) do
-      {:ok, {state_set, room}} -> {:reply, :ok, %{state | state_set: state_set, room: room}}
+    case Repo.transaction(insert_single_event(room, state_set, leave_event)) do
+      {:ok, {state_set, room, _}} -> {:reply, :ok, %{state | state_set: state_set, room: room}}
       {:error, reason} -> {:reply, {:error, reason}, state}
     end
   end
@@ -286,8 +305,8 @@ defmodule MatrixServer.RoomServer do
       ) do
     kick_event = Event.Kick.new(room, account, user_id, reason)
 
-    case insert_single_event(room, state_set, kick_event) do
-      {:ok, {state_set, room}} -> {:reply, :ok, %{state | state_set: state_set, room: room}}
+    case Repo.transaction(insert_single_event(room, state_set, kick_event)) do
+      {:ok, {state_set, room, _}} -> {:reply, :ok, %{state | state_set: state_set, room: room}}
       {:error, reason} -> {:reply, {:error, reason}, state}
     end
   end
@@ -299,8 +318,8 @@ defmodule MatrixServer.RoomServer do
       ) do
     ban_event = Event.Ban.new(room, account, user_id, reason)
 
-    case insert_single_event(room, state_set, ban_event) do
-      {:ok, {state_set, room}} -> {:reply, :ok, %{state | state_set: state_set, room: room}}
+    case Repo.transaction(insert_single_event(room, state_set, ban_event)) do
+      {:ok, {state_set, room, _}} -> {:reply, :ok, %{state | state_set: state_set, room: room}}
       {:error, reason} -> {:reply, {:error, reason}, state}
     end
   end
@@ -308,8 +327,8 @@ defmodule MatrixServer.RoomServer do
   def handle_call({:unban, account, user_id}, _from, %{room: room, state_set: state_set} = state) do
     unban_event = Event.Unban.new(room, account, user_id)
 
-    case insert_single_event(room, state_set, unban_event) do
-      {:ok, {state_set, room}} -> {:reply, :ok, %{state | state_set: state_set, room: room}}
+    case Repo.transaction(insert_single_event(room, state_set, unban_event)) do
+      {:ok, {state_set, room, _}} -> {:reply, :ok, %{state | state_set: state_set, room: room}}
       {:error, reason} -> {:reply, {:error, reason}, state}
     end
   end
@@ -334,18 +353,64 @@ defmodule MatrixServer.RoomServer do
     end
   end
 
-  @spec insert_single_event(Room.t(), t(), Event.t()) :: {:ok, {t(), Room.t()}} | {:error, atom()}
+  def handle_call(
+        {:send_message, account, device, event_type, content, txn_id},
+        _from,
+        %{room: room, state_set: state_set} = state
+      ) do
+    message_event = Event.custom_message(room, account, event_type, content)
+
+    case Repo.transaction(insert_custom_message(state_set, room, device, message_event, txn_id)) do
+      {:ok, {state_set, room, event_id}} ->
+        {:reply, {:ok, event_id}, %{state | state_set: state_set, room: room}}
+
+      {:error, reason} ->
+        {:reply, {:error, reason}, state}
+    end
+  end
+
+  defp insert_custom_message(
+         state_set,
+         room,
+         %Device{id: device_id} = device,
+         message_event,
+         txn_id
+       ) do
+    fn ->
+      # Check if we already executed this transaction.
+      case Repo.one(
+             from dt in DeviceTransaction,
+               where: dt.txn_id == ^txn_id and dt.device_id == ^device_id
+           ) do
+        %DeviceTransaction{event_id: event_id} ->
+          {state_set, room, event_id}
+
+        nil ->
+          with {state_set, room, %Event{event_id: event_id}} <-
+                 insert_single_event(room, state_set, message_event).() do
+            # Mark this transaction as done.
+            Ecto.build_assoc(device, :device_transactions, txn_id: txn_id, event_id: event_id)
+            |> Repo.insert!()
+
+            {state_set, room, event_id}
+          end
+      end
+    end
+  end
+
+  @spec insert_single_event(Room.t(), t(), Event.t()) ::
+          (() -> {t(), Room.t(), Event.t()} | {:error, atom()})
   defp insert_single_event(room, state_set, event) do
-    Repo.transaction(fn ->
+    fn ->
       case finalize_and_insert_event(event, state_set, room) do
-        {:ok, state_set, room} ->
+        {:ok, state_set, room, event} ->
           _ = update_room_state_set(room, state_set)
-          {state_set, room}
+          {state_set, room, event}
 
         {:error, reason} ->
           Repo.rollback(reason)
       end
-    end)
+    end
   end
 
   # Get a function that inserts all events for room creation.
@@ -374,7 +439,7 @@ defmodule MatrixServer.RoomServer do
       result =
         Enum.reduce_while(events, {%{}, room}, fn event, {state_set, room} ->
           case finalize_and_insert_event(event, state_set, room) do
-            {:ok, state_set, room} -> {:cont, {state_set, room}}
+            {:ok, state_set, room, _} -> {:cont, {state_set, room}}
             {:error, reason} -> {:halt, {:error, reason}}
           end
         end)
@@ -441,7 +506,7 @@ defmodule MatrixServer.RoomServer do
   # - Event ID
   # - Signature
   @spec finalize_and_insert_event(Event.t(), t(), Room.t()) ::
-          {:ok, t(), Room.t()} | {:error, atom()}
+          {:ok, t(), Room.t(), Event.t()} | {:error, atom()}
   defp finalize_and_insert_event(
          event,
          state_set,
@@ -512,7 +577,7 @@ defmodule MatrixServer.RoomServer do
   # Implements the checks as described in the
   # [Matrix docs](https://matrix.org/docs/spec/server_server/latest#checks-performed-on-receipt-of-a-pdu).
   @spec authenticate_and_insert_event(Event.t(), t(), Room.t()) ::
-          {:ok, t(), Room.t()} | {:error, atom()}
+          {:ok, t(), Room.t(), Event.t()} | {:error, atom()}
   defp authenticate_and_insert_event(event, current_state_set, room) do
     # TODO: Correctly handle soft fails.
     # Check the following things:
@@ -533,7 +598,7 @@ defmodule MatrixServer.RoomServer do
       # TODO: Do this as a background job, and not after every insert...
       _ = update_joined_rooms(room, state_set)
 
-      {:ok, state_set, room}
+      {:ok, state_set, room, event}
     else
       _ -> {:error, :authorization}
     end
diff --git a/lib/matrix_server/schema/device.ex b/lib/matrix_server/schema/device.ex
index 3e1a6ba..0c8b80b 100644
--- a/lib/matrix_server/schema/device.ex
+++ b/lib/matrix_server/schema/device.ex
@@ -3,7 +3,7 @@ defmodule MatrixServer.Device do
 
   import Ecto.{Changeset, Query}
 
-  alias MatrixServer.{Account, Device, Repo}
+  alias MatrixServer.{Account, Device, Repo, DeviceTransaction}
   alias MatrixServerWeb.Client.Request.Login
 
   @type t :: %__MODULE__{
@@ -19,6 +19,7 @@ defmodule MatrixServer.Device do
     field :display_name, :string
 
     belongs_to :account, Account
+    has_many :device_transactions, DeviceTransaction
   end
 
   def changeset(device, params \\ %{}) do
diff --git a/lib/matrix_server/schema/device_transaction.ex b/lib/matrix_server/schema/device_transaction.ex
new file mode 100644
index 0000000..5d2d4a9
--- /dev/null
+++ b/lib/matrix_server/schema/device_transaction.ex
@@ -0,0 +1,18 @@
+defmodule MatrixServer.DeviceTransaction do
+  use Ecto.Schema
+
+  alias MatrixServer.Device
+
+  @type t :: %__MODULE__{
+          txn_id: String.t(),
+          event_id: String.t(),
+          device_id: integer()
+        }
+
+  @primary_key {:txn_id, :string, []}
+  schema "device_transactions" do
+    field :event_id, :string
+
+    belongs_to :device, Device
+  end
+end
diff --git a/lib/matrix_server/schema/event.ex b/lib/matrix_server/schema/event.ex
index 798e050..77b7000 100644
--- a/lib/matrix_server/schema/event.ex
+++ b/lib/matrix_server/schema/event.ex
@@ -10,7 +10,7 @@ defmodule MatrixServer.Event do
   @type t :: %__MODULE__{
           type: String.t(),
           origin_server_ts: integer(),
-          state_key: String.t(),
+          state_key: String.t() | nil,
           sender: UserId.t(),
           content: map(),
           prev_events: [String.t()] | nil,
@@ -73,6 +73,15 @@ defmodule MatrixServer.Event do
     }
   end
 
+  @spec custom_message(Room.t(), Account.t(), String.t(), map()) :: t()
+  def custom_message(room, sender, type, content) do
+    %Event{
+      Event.new(room, sender)
+      | type: type,
+        content: content
+    }
+  end
+
   @spec is_control_event(t()) :: boolean()
   def is_control_event(%Event{type: "m.room.power_levels", state_key: ""}), do: true
   def is_control_event(%Event{type: "m.room.join_rules", state_key: ""}), do: true
@@ -131,7 +140,7 @@ defmodule MatrixServer.Event do
       length(prev_events) == length(prev_event_ids) and
       not MatrixServer.has_duplicates?(state_pairs) and
       valid_auth_events?(event, auth_events) and
-      Enum.find_value(state_pairs, &(&1 == {"m.room.create", ""})) and
+      Enum.find_value(state_pairs, false, &(&1 == {"m.room.create", ""})) and
       do_prevalidate(event, auth_events, prev_events)
   end
 
diff --git a/lib/matrix_server/state_resolution/authorization.ex b/lib/matrix_server/state_resolution/authorization.ex
index b8e83b5..6559ff2 100644
--- a/lib/matrix_server/state_resolution/authorization.ex
+++ b/lib/matrix_server/state_resolution/authorization.ex
@@ -1,6 +1,6 @@
 defmodule MatrixServer.StateResolution.Authorization do
   @moduledoc """
-  Implementation of Matrix event authorization rules for stat resolution.
+  Implementation of Matrix event authorization rules for state resolution.
 
   Note that some authorization rules are already checked in
   `MatrixServer.Event.prevalidate/1` so they are skipped here.
@@ -156,9 +156,14 @@ defmodule MatrixServer.StateResolution.Authorization do
 
     # Check rules: 8, 9
     cond do
-      not has_power_level?(to_string(sender), power_levels, {:event, event}) -> false
-      String.starts_with?(state_key, "@") and state_key != sender -> false
-      true -> __authorized?(event, state_set)
+      not has_power_level?(to_string(sender), power_levels, {:event, event}) ->
+        false
+
+      is_binary(state_key) and String.starts_with?(state_key, "@") and state_key != sender ->
+        false
+
+      true ->
+        __authorized?(event, state_set)
     end
   end
 
diff --git a/lib/matrix_server_web/client/controllers/room_controller.ex b/lib/matrix_server_web/client/controllers/room_controller.ex
index 38f5a35..e9cc79a 100644
--- a/lib/matrix_server_web/client/controllers/room_controller.ex
+++ b/lib/matrix_server_web/client/controllers/room_controller.ex
@@ -208,13 +208,28 @@ defmodule MatrixServerWeb.Client.RoomController do
 
   def unban(conn, _), do: put_error(conn, :missing_param)
 
-  def send_message(%Conn{assigns: %{account: account}, body_params: body_params} = conn, %{
-        "room_id" => room_id,
-        "event_type" => event_type,
-        "txn_id" => txn_id
-      }) do
-    conn
-    |> send_resp(200, [])
-    |> halt()
+  def send_message(
+        %Conn{assigns: %{account: account, device: device}, body_params: body_params} = conn,
+        %{
+          "room_id" => room_id,
+          "event_type" => event_type,
+          "txn_id" => txn_id
+        }
+      ) do
+    case RoomServer.get_room_server(room_id) do
+      {:ok, pid} ->
+        case RoomServer.send_message(pid, account, device, event_type, body_params, txn_id) do
+          {:ok, event_id} ->
+            conn
+            |> put_status(200)
+            |> json(%{event_id: event_id})
+
+          {:error, _} ->
+            put_error(conn, :unknown)
+        end
+
+      {:error, :not_found} ->
+        put_error(conn, :not_found, "The given room was not found.")
+    end
   end
 end
diff --git a/priv/repo/migrations/20210830160818_create_initial_tables.exs b/priv/repo/migrations/20210830160818_create_initial_tables.exs
index b67cd55..9884e2c 100644
--- a/priv/repo/migrations/20210830160818_create_initial_tables.exs
+++ b/priv/repo/migrations/20210830160818_create_initial_tables.exs
@@ -63,7 +63,7 @@ defmodule MatrixServer.Repo.Migrations.CreateInitialTables do
 
     create table(:devices) do
       add :device_id, :string, null: false
-      add :access_token, :string
+      add :access_token, :string, null: false
       add :display_name, :string
 
       add :account_id, references(:accounts, on_delete: :delete_all), null: false
@@ -72,5 +72,11 @@ defmodule MatrixServer.Repo.Migrations.CreateInitialTables do
     create index(:devices, [:device_id, :account_id], unique: true)
     create index(:devices, [:account_id])
     create index(:devices, [:access_token], unique: true)
+
+    create table(:device_transactions, primary_key: false) do
+      add :txn_id, :string, primary_key: true, null: false
+      add :device_id, references(:devices, on_delete: :delete_all), primary_key: true, null: false
+      add :event_id, :string, null: false
+    end
   end
 end
diff --git a/priv/repo/seeds.exs b/priv/repo/seeds.exs
index ef17929..527b63f 100644
--- a/priv/repo/seeds.exs
+++ b/priv/repo/seeds.exs
@@ -1,12 +1,15 @@
-alias MatrixServer.{Repo, Account, Device}
+alias MatrixServer.{Repo, Account}
 
-Repo.insert!(%Account{
-  localpart: "chuck",
-  password_hash: Bcrypt.hash_pwd_salt("sneed")
-})
+account =
+  Repo.insert!(%Account{
+    localpart: "chuck",
+    password_hash: Bcrypt.hash_pwd_salt("sneed")
+  })
 
-Repo.insert(%Device{
+account
+|> Ecto.build_assoc(:devices,
   device_id: "android",
   display_name: "My Android",
-  localpart: "chuck"
-})
+  access_token: "sneed"
+)
+|> Repo.insert!()