Needed to resurrect validation, too.

core/src/main/java/jenkins/model/JenkinsLocationConfiguration.java

@@ -3,9 +3,13 @@ package jenkins.model;
 import hudson.Extension;
 import hudson.Util;
 import hudson.XmlFile;
+import hudson.util.FormValidation;
 import net.sf.json.JSONObject;
+import org.kohsuke.stapler.QueryParameter;
 import org.kohsuke.stapler.StaplerRequest;
 
+import javax.mail.internet.AddressException;
+import javax.mail.internet.InternetAddress;
 import java.io.File;
 import java.io.IOException;
 import java.util.logging.Level;
@@ -84,5 +88,23 @@ public class JenkinsLocationConfiguration extends GlobalConfiguration {
         return true;
     }
 
+    /**
+     * Checks the URL in <tt>global.jelly</tt>
+     */
+    public FormValidation doCheckUrl(@QueryParameter String value) {
+        if(value.startsWith("http://localhost"))
+            return FormValidation.warning(Messages.Mailer_Localhost_Error());
+        return FormValidation.ok();
+    }
+
+    public FormValidation doCheckAdminAddress(@QueryParameter String value) {
+        try {
+            new InternetAddress(value);
+            return FormValidation.ok();
+        } catch (AddressException e) {
+            return FormValidation.error(e.getMessage());
+        }
+    }
+
     private static final Logger LOGGER = Logger.getLogger(JenkinsLocationConfiguration.class.getName());
 }

core/src/main/resources/jenkins/model/Messages.properties

@@ -62,6 +62,7 @@ CLI.safe-shutdown.shortDescription=Puts Jenkins into the quiet mode, wait for ex
 DefaultProjectNamingStrategy.DisplayName=Default
 
 Mailer.Address.Not.Configured=address not configured yet <nobody@nowhere>
+Mailer.Localhost.Error=Please set a valid host name, instead of localhost
 
 PatternProjectNamingStrategy.DisplayName=Pattern
 PatternProjectNamingStrategy.NamePatternRequired=Name Pattern is required

core/src/main/resources/jenkins/model/Messages_da.properties

@@ -51,3 +51,4 @@ CLI.reload-configuration.shortDescription=Genindl\u00e6s alle data fra filsystem
 Nyttigt hvis du har modificeret konfigurationsfiler direkte, udenom Jenkins.
 
 Mailer.Address.Not.Configured=Adresse ikke konfigureret <nobody@nowhere>
+Mailer.Localhost.Error=Venligst inds\u00e6t et gyldigt v\u00e6rtsnavn, istedet for localhost

core/src/main/resources/jenkins/model/Messages_de.properties

@@ -53,3 +53,4 @@ CLI.reload-configuration.shortDescription=Alle Daten im Speicher verwerfen und K
 CLI.safe-restart.shortDescription=Startet Jenkins neu.
 
 Mailer.Address.Not.Configured=Adresse nicht konfiguriert <nobody@nowhere>
+Mailer.Localhost.Error=Bitte verwenden Sie einen konkreten Hostnamen anstelle von ''localhost''.

core/src/main/resources/jenkins/model/Messages_es.properties

@@ -61,3 +61,4 @@ CauseOfInterruption.ShortDescription=Cancelado por {0}
 PatternProjectNamingStrategy.NamePatternInvalidSyntax=La sintaxis de la expresin regular no es vlida
 
 Mailer.Address.Not.Configured=Direccin no configurada todava <nobody@nowhere>
+Mailer.Localhost.Error=Escriba un nombre de servidor correcto en lugar de "localhost"

core/src/main/resources/jenkins/model/Messages_ja.properties

@@ -61,3 +61,4 @@ PatternProjectNamingStrategy.NamePatternRequired=\u30d1\u30bf\u30fc\u30f3\u306f\
 PatternProjectNamingStrategy.NamePatternInvalidSyntax=\u6b63\u898f\u8868\u73fe\u306b\u8aa4\u308a\u304c\u3042\u308a\u307e\u3059\u3002
 
 Mailer.Address.Not.Configured=\u307e\u3060\u30a2\u30c9\u30ec\u30b9\u304c\u8a2d\u5b9a\u3055\u308c\u3066\u3044\u307e\u305b\u3093\u3002 <nobody@nowhere>
+Mailer.Localhost.Error=localhost\u306e\u4ee3\u308f\u308a\u306b\u59a5\u5f53\u306a\u30db\u30b9\u30c8\u540d\u3092\u8a2d\u5b9a\u3057\u3066\u304f\u3060\u3055\u3044\u3002

core/src/main/resources/jenkins/model/Messages_pt_BR.properties

@@ -46,3 +46,5 @@ CLI.keep-build.shortDescription=Marcar a constru\u00e7\u00e3o como permanente
 
 # address not configured yet <nobody@nowhere>
 Mailer.Address.Not.Configured=Endere\u00e7o ainda n\u00e3o configurado <nobody@nowhere>
+# Please set a valid host name, instead of localhost
+Mailer.Localhost.Error=Por favor insira uma nome de host v\u00e1lido.

test/src/test/java/jenkins/model/JenkinsLocationConfigurationTest.java

+package jenkins.model;
+
+import com.gargoylesoftware.htmlunit.html.HtmlInput;
+import com.gargoylesoftware.htmlunit.html.HtmlPage;
+import org.jvnet.hudson.test.HudsonTestCase;
+
+/**
+ * @author Kohsuke Kawaguchi
+ */
+public class JenkinsLocationConfigurationTest extends HudsonTestCase {
+    /**
+     * Makes sure the use of "localhost" in the Hudson URL reports a warning.
+     */
+    public void testLocalHostWarning() throws Exception {
+        HtmlPage p = new WebClient().goTo("configure");
+        HtmlInput url = p.getFormByName("config").getInputByName("_.url");
+        url.setValueAttribute("http://localhost:1234/");
+        assertTrue(p.getDocumentElement().getTextContent().contains("instead of localhost"));
+    }
+}