The warning comes in the form of the "Deposit Limit" above the input estimate edit box. This value shows the remaining amount you can transact with us for the day at your current allowed daily transaction volume.
If you put in an estimated amount above this value, the deposit limit will turn red to indicate the amount you're planning to send would exceed your daily limit. For example, in the screenshot below the user is asking for an estimate for 7 ETH which exceeds his remaining daily limit of 6.887 ETH).
