diff --git a/apps/kolab_guam/src/kolab_guam_session.erl b/apps/kolab_guam/src/kolab_guam_session.erl
--- a/apps/kolab_guam/src/kolab_guam_session.erl
+++ b/apps/kolab_guam/src/kolab_guam_session.erl
@@ -143,6 +143,18 @@
                     end
             end
     end;
+update_split_command_state(Data, #state{ command_split_reset_trigger = reset_for_next_client_command, current_command_split = CurrentCommandSplit }) ->
+    %If we have a currently active command (CurrentCommandSplit), then we reset only if we find a new command (and not just on any new line).
+    %This is relevant for multi-line commands such as APPEND, which follow up with a continuation which we want to avoid buffering.
+    case CurrentCommandSplit of
+        undefined -> undefined;
+        _ ->
+            case eimap_utils:split_command_into_components(Data) of
+                { <<>>, <<>>, <<>> } -> CurrentCommandSplit;
+                { _Tag, <<>>, <<>> } -> CurrentCommandSplit;
+                { _Tag, Command, _Data } -> undefined
+            end
+    end;
 update_split_command_state(_Data, _State) ->
     undefined.
 
@@ -184,15 +196,9 @@
     end.
 
 
-process_preprocessed_client_data(Socket, PreprocessData, #state{ command_split_reset_trigger = CurrentCommandSplitResetTrigger, current_command_split = CurrentCommandSplit } = State) ->
-    NewSplitCommand = case CurrentCommandSplitResetTrigger of
-        reset_for_next_client_command -> undefined;
-        _ -> CurrentCommandSplit
-    end,
-
-    % TODO Maybe we should prepend buffered data here instead.
+process_preprocessed_client_data(Socket, PreprocessData, State) ->
     {Line, Remainder} = read_line(PreprocessData),
-    NewState = process_client_line(Line, Socket, State#state{ current_command_split = NewSplitCommand }),
+    NewState = process_client_line(Line, Socket, State),
 
     case Remainder of
         <<>> -> NewState;
@@ -223,7 +229,9 @@
             eimap:compress(ImapSession), % TODO: make optional
             { TLS, Socket, NewInflator, NewDeflator, UndecidedRules, ActiveRules, <<>>, undefined, undefined };
         nochange ->
-            { ModifiedData, NewSplitCommand, NewSplitResetTrigger, NewUndecidedRules, NewActiveRules, PostAction } = apply_ruleset_clientside(ImapSession, Socket, Data, CurrentCommandSplit, UndecidedRules, ActiveRules),
+            %We  first check if we have to reset the split command before we process the new command in apply_ruleset_clientside
+            NewSplit = update_split_command_state(Data, State),
+            { ModifiedData, NewSplitCommand, NewSplitResetTrigger, NewUndecidedRules, NewActiveRules, PostAction } = apply_ruleset_clientside(ImapSession, Socket, Data, NewSplit, UndecidedRules, ActiveRules),
             BufferThisData =
             case PostAction of
                 perform_passthrough ->
diff --git a/apps/kolab_guam/test/kolab_guam_session_SUITE.erl b/apps/kolab_guam/test/kolab_guam_session_SUITE.erl
--- a/apps/kolab_guam/test/kolab_guam_session_SUITE.erl
+++ b/apps/kolab_guam/test/kolab_guam_session_SUITE.erl
@@ -91,6 +91,29 @@
      }} = kolab_guam_session:handle_info({tcp, Socket, <<"y1 ID (\"name\" \"Test\")\r\n">>}, State#state{ rules_deciding = ActiveRules }),
 
 
+    % Append
+    {noreply, #state{
+      server_config = ServerConfig,
+      buffered_client_data = <<>>,
+      current_command_split = {<<"y1">>,<<"APPEND">>,<<"INBOX {7}">>},
+      command_split_reset_trigger = reset_for_next_client_command
+     } = IntermediateState4} = kolab_guam_session:handle_info({tcp, Socket, <<"y1 APPEND INBOX {7}\r\n123">>}, State#state{ rules_deciding = ActiveRules }),
+
+    {noreply, #state{
+      server_config = ServerConfig,
+      buffered_client_data = <<>>, %This ensures we aren't buffering during the continuation
+      current_command_split = {<<"y1">>,<<"APPEND">>,<<"INBOX {7}">>},
+      command_split_reset_trigger = reset_for_next_client_command
+     } = IntermediateState5} = kolab_guam_session:handle_info({tcp, Socket, <<"456">>}, IntermediateState4),
+
+    {noreply, #state{
+      server_config = ServerConfig,
+      buffered_client_data = <<>>,
+      current_command_split = {<<"y2">>,<<"ENABLE">>,<<"QRESYNC">>},
+      command_split_reset_trigger = reset_for_next_client_command
+     }} = kolab_guam_session:handle_info({tcp, Socket, <<"7\r\ny2 ENABLE QRESYNC\r\n">>}, IntermediateState5),
+
+
     % Don't activate filtering
     {noreply, #state{
       server_config = ServerConfig,