As described under extended namespaces, a message stanza MAY contain any properly-namespaced child element.
In accordance with the default namespace declaration, by default a message stanza is qualified by the 'jabber:client' or 'jabber:server' namespace, which defines certain allowable children of message stanzas. If the message stanza is of type "error", it MUST include an <error/> child; for details, see [XMPP-CORE]. Otherwise, the message stanza MAY contain any of the following child elements without an explicit namespace declaration:
<subject/>
<body/>
<thread/>

Explanation/Reference: https://developer.cisco.com/docs/cisco-unified-communications-single-sign-on-tutorial/#!cisco-unified-communications-single-sign-on-tutorial/learning-more-about-sso