When building on the OSX slave, the build will get to the point where the console says that it is sending email, and it stays there. If you look at the list of executors, the progress bar shows that the build is still on progress. If you look at the job's status page, it looks like the job has completed, and it is no longer running. Clicking on the "Stop build" button next to the job in the "Build Executors Status" section, does nothing. The only way to clear the executor is to restart Jenkins.