 SCP-521 is a red post box. When a letter is inserted through the letter slot of SCP-521, the anomalous effects of the object will activate. No other items inserted into SCP-521 will cause activation of the object. All letters must be addressed from one subject to another directly. Letters sent from a singular subject to a company or representative of a company do not cause SCP-521 activation, or vice versa. Up to 24 hours after time of posting, the posted item will arrive at its destination. However, the original letter posted will not be found inside the envelope, instead replaced with an instance of SCP-521-X.