To check whether the 360 collection letter has been sent back to the household registration, you can contact the 360 collection department by telephone, email, online customer service, etc., provide relevant information and ask if they have been sent back to the household registration. They can check the records in the system and notify the sending status of letters. If you know the mailing method and mailing date of the reminder letter, you can use the online inquiry system of the mailing service provider, such as China Post and SF Express, and enter relevant information (such as mailing number) to track the mailing status. These mail service providers usually provide real-time mail tracking function, and you can check the delivery progress and latest status of letters through the inquiry system.