I consistently get this error when scp plugin tries to upload:

Recording fingerprints
Sending e-mails to: <me>
[SCP] Connecting to <hostname>
ERROR: Failed to upload files
com.jcraft.jsch.JSchException: Auth fail
at com.jcraft.jsch.Session.connect(Session.java:473)
at com.jcraft.jsch.Session.connect(Session.java:145)
at be.certipost.hudson.plugin.SCPSite.createSession(SCPSite.java:149)
at be.certipost.hudson.plugin.SCPRepositoryPublisher.perform(SCPRepositoryPublisher.java:106)
at hudson.tasks.BuildStepMonitor$3.perform(BuildStepMonitor.java:36)
at hudson.model.AbstractBuild$AbstractRunner.perform(AbstractBuild.java:601)
at hudson.model.AbstractBuild$AbstractRunner.performAllBuildSteps(AbstractBuild.java:580)
at hudson.model.AbstractBuild$AbstractRunner.performAllBuildSteps(AbstractBuild.java:558)
at hudson.model.Build$RunnerImpl.post2(Build.java:158)
at hudson.model.AbstractBuild$AbstractRunner.post(AbstractBuild.java:528)
at hudson.model.Run.run(Run.java:1296)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:129)
Finished: UNSTABLE

This only happens when uploading to a host that's behind a firewall. It can succesfully upload to a host on the same network as the Hudson master and slave.
I can manually, using "system" scp, upload files to the host through the firewall with no problems.

The "auth.log" file on the remote host is not touched by Hudson's attempts to upload, indicating that it's not the host itself that denies access.