官方文档note很清楚的,
Example 3. A content message with receipt requested
<message
from='northumberland@shakespeare.lit/westminster'
id='richard2-4.1.247'
to='kingrichard@royalty.england.lit/throne'>
<body>My lord, dispatch; read o'er these articles.</body>
<requestxmlns='urn:xmpp:receipts'/>
</message>
Note: A sender MUST include an 'id' attribute on every content message that requests a receipt, so that the sender can properly track ack messages.