SearchDocument returns unkown Document error

[Niades]

Hello!
I am trying to send the following request to the Zimbra server:

POST /service/soap HTTP/1.1
SOAPAction: ""
Content-Type: text/xml;charset="utf-8"
Accept: text/xml
Host: 192.168.138.133
Content-Length: 514
Expect: 100-continue

<soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope">
  <soap:Header>
    <authToken>%token acquired from AuthRequest%</authToken>
    <context xmlns="urn:zimbra" />
  </soap:Header>
  <soap:Body>
    <SearchRequest xlmns="urn:zimbraMail">
      <query>is:unread</query>
    </SearchRequest>
  </soap:Body>
</soap:Envelope>

What am I doing wrong?
Thanks in advance.