Generally speaking, you can write a letter after you know the detailed address (but the audit is strict). If there is something urgent, you should take it to the suspect immediately. If you have entrusted a defense lawyer, you can ask the lawyer to take the message directly. If you don't have an attorney, you can. Please refer to the following laws and regulations:
According to the Measures for the Implementation of the Regulations of People's Republic of China (PRC) Detention Center,
Article 39 When meeting a prisoner, a lawyer must hold a work permit of a law firm (or legal advisory office) and a special letter of introduction in a fixed format; Other defenders who want to meet with detainees must hold a special letter of introduction from the people's court.
The Measures for the Implementation of the Regulations of People's Republic of China (PRC) Detention Center states:
Article 36 With the consent of the case-handling organ and the approval of the competent bureau and director of the public security organ at or above the county level, a criminal may temporarily leave his dying spouse, parents or children. Prisoners who are seriously ill and cannot return to their original places that day are not allowed to visit.
Visitors shall be accompanied and supervised by two or more investigators, and shall not spend the night outside.
Article 37 The articles for daily use, books and periodicals for study given or sent by close relatives of criminals shall be registered in detail in the detention center, and shall be handed over to criminals after inspection and acceptance. Cash is kept by the detention center and can be paid by prisoners when they have a legitimate purpose; Articles that are not allowed to be brought in shall be returned on the spot or after registration.
Prisoners in detention centers can take messages to their families. Article 28 "Regulations on Detention Center" Criminals may communicate and meet with their close relatives with the consent of the case-handling organ and the approval of the public security organ during their detention. Article 29 If a close relative of a criminal is seriously ill or dies, he shall promptly notify the criminal. When a prisoner's spouse, parents or children are critically ill, except for those who are seriously ill, with the consent of the case-handling organ and the approval of the public security organ, prisoners are allowed to visit at home under strict supervision. Article 30 prison guards must inspect articles given to prisoners by their close relatives. Article 31 A detention center, entrusted by the case-handling organ, may inspect the letters sent and received by criminals. Anyone who is found to obstruct investigation, prosecution or trial may be detained and transferred to the case-handling organ for handling. Article 32 In a case where a people's procuratorate decides to initiate a public prosecution, a prisoner in custody may, after receiving a copy of the indictment, meet and correspond with a defender entrusted by himself or a defender designated by the people's court.