The other party's phone is turned off, but I can successfully send a message to the other party. What happened?

Just because you sent it successfully does not mean that the other party has successfully received it at that time. The short message you send will be stored in the server of the short message center first, and the system will keep trying to send it to the other party within the validity period (48 hours for ordinary point-to-point short messages). If the other party's number is normal, it will be received normally. If it is abnormal and exceeds the validity period, the short message will be deleted in the short message center and will not be sent again. At the same time, your number will not be charged.