SSLException: Received fatal alert: certificate_unknown - what does it mean?

I am getting the "javax.net.ssl.SSLException: Received fatal alert: certificate_unknown" error when attempting to send a HTTPS message.

I believe the message is due to the endpoint not recognising the sent certificate or is rejecting it. But I want to be doubly sure this is a server issue and not a local client issue before I go barking up the wrong tree with my error analysis.

Can you guys please clarify the meaning of this error (google isn't helping much) and whether it is client or endpoint server related.

FYI: I am using Websphere Application Server 6.1

Edit: The stack trace is:

uk.gov.dwp.ifa.services.jef.CommandException:  javax.net.ssl.SSLException: Received fatal alert: certificate_unknown
    at com.sun.net.ssl.internal.ssl.SSLSocketImpl.b(DashoA6275)
    at com.sun.net.ssl.internal.ssl.SSLSocketImpl.a(DashoA6275)
    at com.sun.net.ssl.internal.ssl.SSLSocketImpl.a(DashoA6275)
    at com.sun.net.ssl.internal.ssl.AppOutputStream.write(DashoA6275)
    at java.io.BufferedOutputStream.flushBuffer(BufferedOutputStream.java:65)
    at java.io.BufferedOutputStream.flush(BufferedOutputStream.java:123)
    at org.apache.commons.httpclient.methods.EntityEnclosingMethod.writeRequestBody(EntityEnclosingMethod.java:502)
    at org.apache.commons.httpclient.HttpMethodBase.writeRequest(HttpMethodBase.java:1973)
    at org.apache.commons.httpclient.HttpMethodBase.execute(HttpMethodBase.java:993)
    at org.apache.commons.httpclient.HttpMethodDirector.executeWithRetry(HttpMethodDirector.java:393)
    at org.apache.commons.httpclient.HttpMethodDirector.executeMethod(HttpMethodDirector.java:168)
    at org.apache.commons.httpclient.HttpClient.executeMethod(HttpClient.java:396)
    at org.apache.commons.httpclient.HttpClient.executeMethod(HttpClient.java:324)
    at uk.gov.dwp.ifa.services.jef.commands.HttpRequestCommand.postMessage(HttpRequestCommand.java:406)
    at uk.gov.dwp.ifa.services.jef.commands.HttpRequestCommand.evaluate(HttpRequestCommand.java:288)
    at uk.gov.dwp.ifa.services.jef.AbstractController.evaluate(AbstractController.java:46)
    at uk.gov.dwp.ifa.services.jef.SequentialAggregationController.evaluate(SequentialAggregationController.java:30)
    at uk.gov.dwp.ifa.services.jef.FlowMaster.evaluate(FlowMaster.java:86)
    at uk.gov.dwp.ifa.services.inbound.lite.servlet.InboundServlet.processRequest(InboundServlet.java:673)
    at uk.gov.dwp.ifa.services.inbound.lite.servlet.InboundServlet.doPost(InboundServlet.java:232)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:763)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:856)
    at com.ibm.ws.webcontainer.servlet.ServletWra开发者_运维知识库pper.service(ServletWrapper.java:989)
    at com.ibm.ws.webcontainer.servlet.ServletWrapper.service(ServletWrapper.java:930)
    at com.ibm.ws.webcontainer.filter.WebAppFilterChain.doFilter(WebAppFilterChain.java:118)
    at com.ibm.ws.webcontainer.filter.WebAppFilterChain._doFilter(WebAppFilterChain.java:87)
    at com.ibm.ws.webcontainer.filter.WebAppFilterManager.doFilter(WebAppFilterManager.java:761)
    at com.ibm.ws.webcontainer.filter.WebAppFilterManager.doFilter(WebAppFilterManager.java:673)
    at com.ibm.ws.webcontainer.servlet.ServletWrapper.handleRequest(ServletWrapper.java:498)
    at com.ibm.ws.wswebcontainer.servlet.ServletWrapper.handleRequest(ServletWrapper.java:464)
    at com.ibm.ws.webcontainer.servlet.CacheServletWrapper.handleRequest(CacheServletWrapper.java:90)
    at com.ibm.ws.webcontainer.WebContainer.handleRequest(WebContainer.java:744)
    at com.ibm.ws.wswebcontainer.WebContainer.handleRequest(WebContainer.java:1433)
    at com.ibm.ws.webcontainer.channel.WCChannelLink.ready(WCChannelLink.java:112)
    at com.ibm.ws.http.channel.inbound.impl.HttpInboundLink.handleDiscrimination(HttpInboundLink.java:454)
    at com.ibm.ws.http.channel.inbound.impl.HttpInboundLink.handleNewInformation(HttpInboundLink.java:383)
    at com.ibm.ws.http.channel.inbound.impl.HttpInboundLink.ready(HttpInboundLink.java:263)
    at com.ibm.ws.tcp.channel.impl.NewConnectionInitialReadCallback.sendToDiscriminators(NewConnectionInitialReadCallback.java:214)
    at com.ibm.ws.tcp.channel.impl.NewConnectionInitialReadCallback.complete(NewConnectionInitialReadCallback.java:113)
    at com.ibm.ws.tcp.channel.impl.AioReadCompletionListener.futureCompleted(AioReadCompletionListener.java:165)
    at com.ibm.io.async.AbstractAsyncFuture.invokeCallback(AbstractAsyncFuture.java:217)
    at com.ibm.io.async.AsyncChannelFuture.fireCompletionActions(AsyncChannelFuture.java:161)
    at com.ibm.io.async.AsyncFuture.completed(AsyncFuture.java:136)
    at com.ibm.io.async.ResultHandler.complete(ResultHandler.java:195)
    at com.ibm.io.async.ResultHandler.runEventProcessingLoop(ResultHandler.java:743)
    at com.ibm.io.async.ResultHandler$2.run(ResultHandler.java:873)
    at com.ibm.ws.util.ThreadPool$Worker.run(ThreadPool.java:1469)

    at uk.gov.dwp.ifa.services.jef.commands.HttpRequestCommand.evaluate(HttpRequestCommand.java:302)
    at uk.gov.dwp.ifa.services.jef.AbstractController.evaluate(AbstractController.java:46)
    at uk.gov.dwp.ifa.services.jef.SequentialAggregationController.evaluate(SequentialAggregationController.java:30)
    at uk.gov.dwp.ifa.services.jef.FlowMaster.evaluate(FlowMaster.java:86)
    at uk.gov.dwp.ifa.services.inbound.lite.servlet.InboundServlet.processRequest(InboundServlet.java:673)
    at uk.gov.dwp.ifa.services.inbound.lite.servlet.InboundServlet.doPost(InboundServlet.java:232)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:763)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:856)
    at com.ibm.ws.webcontainer.servlet.ServletWrapper.service(ServletWrapper.java:989)
    at com.ibm.ws.webcontainer.servlet.ServletWrapper.service(ServletWrapper.java:930)
    at com.ibm.ws.webcontainer.filter.WebAppFilterChain.doFilter(WebAppFilterChain.java:118)
    at com.ibm.ws.webcontainer.filter.WebAppFilterChain._doFilter(WebAppFilterChain.java:87)
    at com.ibm.ws.webcontainer.filter.WebAppFilterManager.doFilter(WebAppFilterManager.java:761)
    at com.ibm.ws.webcontainer.filter.WebAppFilterManager.doFilter(WebAppFilterManager.java:673)
    at com.ibm.ws.webcontainer.servlet.ServletWrapper.handleRequest(ServletWrapper.java:498)
    at com.ibm.ws.wswebcontainer.servlet.ServletWrapper.handleRequest(ServletWrapper.java:464)
    at com.ibm.ws.webcontainer.servlet.CacheServletWrapper.handleRequest(CacheServletWrapper.java:90)
    at com.ibm.ws.webcontainer.WebContainer.handleRequest(WebContainer.java:744)
    at com.ibm.ws.wswebcontainer.WebContainer.handleRequest(WebContainer.java:1433)
    at com.ibm.ws.webcontainer.channel.WCChannelLink.ready(WCChannelLink.java:112)
    at com.ibm.ws.http.channel.inbound.impl.HttpInboundLink.handleDiscrimination(HttpInboundLink.java:454)
    at com.ibm.ws.http.channel.inbound.impl.HttpInboundLink.handleNewInformation(HttpInboundLink.java:383)
    at com.ibm.ws.http.channel.inbound.impl.HttpInboundLink.ready(HttpInboundLink.java:263)
    at com.ibm.ws.tcp.channel.impl.NewConnectionInitialReadCallback.sendToDiscriminators(NewConnectionInitialReadCallback.java:214)
    at com.ibm.ws.tcp.channel.impl.NewConnectionInitialReadCallback.complete(NewConnectionInitialReadCallback.java:113)
    at com.ibm.ws.tcp.channel.impl.AioReadCompletionListener.futureCompleted(AioReadCompletionListener.java:165)
    at com.ibm.io.async.AbstractAsyncFuture.invokeCallback(AbstractAsyncFuture.java:217)
    at com.ibm.io.async.AsyncChannelFuture.fireCompletionActions(AsyncChannelFuture.java:161)
    at com.ibm.io.async.AsyncFuture.completed(AsyncFuture.java:136)
    at com.ibm.io.async.ResultHandler.complete(ResultHandler.java:195)
    at com.ibm.io.async.ResultHandler.runEventProcessingLoop(ResultHandler.java:743)
    at com.ibm.io.async.ResultHandler$2.run(ResultHandler.java:873)
    at com.ibm.ws.util.ThreadPool$Worker.run(ThreadPool.java:1469)
Caused by: javax.net.ssl.SSLException: Received fatal alert: certificate_unknown
    at com.sun.net.ssl.internal.ssl.SSLSocketImpl.b(DashoA6275)
    at com.sun.net.ssl.internal.ssl.SSLSocketImpl.a(DashoA6275)
    at com.sun.net.ssl.internal.ssl.SSLSocketImpl.a(DashoA6275)
    at com.sun.net.ssl.internal.ssl.AppOutputStream.write(DashoA6275)
    at java.io.BufferedOutputStream.flushBuffer(BufferedOutputStream.java:65)
    at java.io.BufferedOutputStream.flush(BufferedOutputStream.java:123)
    at org.apache.commons.httpclient.methods.EntityEnclosingMethod.writeRequestBody(EntityEnclosingMethod.java:502)
    at org.apache.commons.httpclient.HttpMethodBase.writeRequest(HttpMethodBase.java:1973)
    at org.apache.commons.httpclient.HttpMethodBase.execute(HttpMethodBase.java:993)
    at org.apache.commons.httpclient.HttpMethodDirector.executeWithRetry(HttpMethodDirector.java:393)
    at org.apache.commons.httpclient.HttpMethodDirector.executeMethod(HttpMethodDirector.java:168)
    at org.apache.commons.httpclient.HttpClient.executeMethod(HttpClient.java:396)
    at org.apache.commons.httpclient.HttpClient.executeMethod(HttpClient.java:324)
    at uk.gov.dwp.ifa.services.jef.commands.HttpRequestCommand.postMessage(HttpRequestCommand.java:406)
    at uk.gov.dwp.ifa.services.jef.commands.HttpRequestCommand.evaluate(HttpRequestCommand.java:288)
    ... 32 more