Why do I automatically change from SMS to MMS every time I send a text message with iphone, and I can't send it out?

Apple mobile phone sends SMS to MMS: When sending SMS, if you enter too much text or content in the subject field of the message, the SMS will be sent to MMS.

The processing method is as follows: Go to Settings > Information, and select "Send and Receive MMS" or "Show Subject Bar" to close as needed. The processing method of short message group sending is the same as single sending.

Note: Turning off the "Send and Receive MMS" function will affect the MMS sending and receiving function of iPhone, so please handle it as appropriate.