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What is the reason why Huawei P9 mobile phone messages are automatically converted into MMS?

This can happen if a picture is inserted or the content can only be sent by MMS. The built-in information function is basically that when you are writing information, press the menu button and click Insert Attachment. Some have the option of having an attachment directly, so you can click it. When you select a picture, it will automatically become a multimedia message. ?