When I send a message from my Apple phone, why does the recipient appear as an email address?

Because of the use of the "imessage" function in Apple's mobile phone.

IMessage is an instant messaging software launched by Apple, which can send short messages, videos and so on. It has high security. Different from the SMS/MMS service of operators, users only need the data support of WiFi or cellular data network to complete communication.

Functional features:

Exchange text, photos and videos at will.

With iMessage, you can chat with anyone who uses iPad, iPhone, iPod touch or runs a lot of people? Mac users of Mountain Lion send and receive messages to each other, and can also send photos, videos, location information and contact information. If you have more than one Apple device, iMessage can keep an uninterrupted conversation on all devices. In addition, you can send text messages, photos and videos to other mobile phones through the cellular network. You can even ask Siri to text you. Just say "Tell Xiaoli I'm on my way" and Siri will help you write and send messages.