If you enter smtp authentication data at "Configure System" and save your
changes you'll get the following exception:

exception

javax.servlet.ServletException: net.sf.json.JSONException:
JSONObject["smtpAuthUserName"] not found.
org.kohsuke.stapler.Stapler.invoke(Stapler.java:499)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:403)
org.kohsuke.stapler.Stapler.service(Stapler.java:116)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:94)
hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:86)
hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:47)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:84)
hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:76)
hudson.security.HudsonFilter.doFilter(HudsonFilter.java:154)

root cause

net.sf.json.JSONException: JSONObject["smtpAuthUserName"] not found.
net.sf.json.JSONObject.getString(JSONObject.java:1767)
hudson.tasks.Mailer$DescriptorImpl.configure(Mailer.java:278)
hudson.model.Hudson.configureDescriptor(Hudson.java:2313)
hudson.model.Hudson.doConfigSubmit(Hudson.java:2249)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
java.lang.reflect.Method.invoke(Method.java:597)
org.kohsuke.stapler.Function$InstanceFunction.invoke(Function.java:185)
org.kohsuke.stapler.Function.bindAndInvoke(Function.java:101)
org.kohsuke.stapler.Function.bindAndInvokeAndServeResponse(Function.java:54)
org.kohsuke.stapler.MetaClass$1.doDispatch(MetaClass.java:74)
org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:487)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:403)
org.kohsuke.stapler.Stapler.service(Stapler.java:116)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:94)
hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:86)
hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:47)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:84)
hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:76)
hudson.security.HudsonFilter.doFilter(HudsonFilter.java:154)