Setting exclusivity test

Author: lukewhiting

x-pack/plugin/watcher/src/test/java/org/elasticsearch/xpack/watcher/notification/email/EmailServiceTests.java

@@ -69,6 +69,31 @@ public void testSend() throws Exception {
         assertThat(sent.account(), is("account1"));
     }
 
+    public void testDomainAndRecipientAllowCantBeSetAtSameTime() {
+        Settings settings = Settings.builder()
+            .putList("xpack.notification.email.account.domain_allowlist", "bar.com")
+            .putList("xpack.notification.email.recipient_allowlist", "*[email protected]")
+            .build();
+
+        IllegalArgumentException e = expectThrows(
+            IllegalArgumentException.class,
+            () -> new EmailService(
+                settings,
+                null,
+                mock(SSLService.class),
+                new ClusterSettings(Settings.EMPTY, new HashSet<>(EmailService.getSettings()))
+            )
+        );
+
+        assertThat(
+            e.getMessage(),
+            containsString(
+                "Cannot set both [xpack.notification.email.recipient_allowlist] and "
+                    + "[xpack.notification.email.account.domain_allowlist] to a non [\"*\"] value at the same time."
+            )
+        );
+    }
+
     public void testAccountSmtpPropertyConfiguration() {
         Settings settings = Settings.builder()
             .put("xpack.notification.email.account.account1.smtp.host", "localhost")