Really sorry here, but I've just tried what you suggested and I'm getting an account.NO_SUCH_ACCOUNT response.
Here's what I'm sending:
<ns0

elegateAuthRequest xmlns:ns0="urn:zimbraAdmin"><account by="id">8695a90b-830a-4838-996f-a32eb3546391</account></ns0

elegateAuthRequest>
The ID is correct. I tried using by="name" with the user@tld and it still failed.
Here is the entire response:
<ns0:Fault xmlns:ns0="http://www.w3.org/2003/05/soap-envelope"><ns0:Code><ns0:Value>soap:Sender</ns0:Value></ns0:Code><ns0:Reason><ns0:Text>no such account: null</ns0:Text></ns0:Reason><ns0

etail><ns1:Error xmlns:ns1="urn:zimbra"><ns1:Code>account.NO_SUCH_A CCOUNT</ns1:Code><ns1:Trace>com.zimbra.cs.account.AccountS erviceException: no such account: null
at com.zimbra.cs.account.AccountServiceException.NO_S UCH_ACCOUNT(AccountServiceException.java:123)
at com.zimbra.cs.service.admin.DelegateAuth.handle(De legateAuth.java:90)
at com.zimbra.soap.SoapEngine.dispatchRequest(SoapEng ine.java:270)
at com.zimbra.soap.SoapEngine.dispatch(SoapEngine.jav a:168)
at com.zimbra.soap.SoapEngine.dispatch(SoapEngine.jav a:90)
at com.zimbra.soap.SoapServlet.doPost(SoapServlet.jav a:223)
at javax.servlet.http.HttpServlet.service(HttpServlet .java:709)
at com.zimbra.cs.servlet.ZimbraServlet.service(Zimbra Servlet.java:162)
at javax.servlet.http.HttpServlet.service(HttpServlet .java:802)
at org.apache.catalina.core.ApplicationFilterChain.in ternalDoFilter(ApplicationFilterChain.java:252)
at org.apache.catalina.core.ApplicationFilterChain.do Filter(ApplicationFilterChain.java:173)
at org.apache.catalina.core.StandardWrapperValve.invo ke(StandardWrapperValve.java:213)
at org.apache.catalina.core.StandardContextValve.invo ke(StandardContextValve.java:178)
at org.apache.catalina.core.StandardHostValve.invoke( StandardHostValve.java:126)
at org.apache.catalina.valves.ErrorReportValve.invoke (ErrorReportValve.java:105)
at org.apache.catalina.core.StandardEngineValve.invok e(StandardEngineValve.java:107)
at org.apache.catalina.valves.AccessLogValve.invoke(A ccessLogValve.java:541)
at org.apache.catalina.connector.CoyoteAdapter.servic e(CoyoteAdapter.java:148)
at org.apache.coyote.http11.Http11Processor.process(H ttp11Processor.java:869)
at org.apache.coyote.http11.Http11BaseProtocol$Http11 ConnectionHandler.processConnection(Http11BaseProt ocol.java:667)
at org.apache.tomcat.util.net.PoolTcpEndpoint.process Socket(PoolTcpEndpoint.java:527)
at org.apache.tomcat.util.net.LeaderFollowerWorkerThr ead.runIt(LeaderFollowerWorkerThread.java:80)
at org.apache.tomcat.util.threads.ThreadPool$ControlR unnable.run(ThreadPool.java:684)
at java.lang.Thread.run(Thread.java:595)
</ns1:Trace></ns1:Error></ns0

etail></ns0:Fault>
Any suggestions?