Why didn't Apple's mobile phone send a receipt and that one has been delivered for reading?

Both read and unread iPhone messages can only be seen by imessage, and iMessage can only be received when the other party's mobile phone is connected to the Internet, so sometimes the receipt cannot be received because there is no internet connection.

Details are as follows:

1. Under the main menu page, click the "Settings" button on the desktop.

2. Click to enter "Information"

3. Just turn on the "Send Read Receipt" switch.