Why does a new SMS sent by iPhone become a new MMS?

IPhone will display MMS only after adding pictures or sending them to an email address (for example, setting the display subject bar of SMS to ON, and writing content in the subject bar in the SMS editing bar is also sending MMS). This situation is caused by the fact that the contacts in your phone book who want to send text messages only have email addresses and no phone numbers. I suggest you add the phone number to the contact's phone book first. When editing a new text message, click Add recipient's contact name, and then click the phone number under the contact name.