No, it will not be executed.

If an execution is rejected (execution fails) on the MT4/5 side, it will also not be executed on the cTrader side.

The reason for this lies in the system linking mechanism between MT4/5 and cTrader.

In this mechanism, an order is first placed on the MT4/5 side. Only when the order is successfully executed in the market (meaning the trade is completed) is the execution information then transmitted to the cTrader side.

Based on the received execution information, the cTrader side immediately executes a similar order as a market order.

Therefore, if an order is rejected on the MT4/5 side (due to slippage, insufficient liquidity, price fluctuations, etc., causing the execution to fail), the order data itself is not transmitted to the cTrader side. As a result, the structure is such that the order will not be executed at all on the cTrader side either.