diff --git a/java/google/registry/reporting/spec11/Spec11EmailUtils.java b/java/google/registry/reporting/spec11/Spec11EmailUtils.java
index 1521409ef..908543ab6 100644
--- a/java/google/registry/reporting/spec11/Spec11EmailUtils.java
+++ b/java/google/registry/reporting/spec11/Spec11EmailUtils.java
@@ -14,12 +14,17 @@
 
 package google.registry.reporting.spec11;
 
+import static com.google.common.base.Strings.isNullOrEmpty;
 import static com.google.common.base.Throwables.getRootCause;
 import static com.google.common.collect.ImmutableList.toImmutableList;
+import static com.google.common.collect.ImmutableSet.toImmutableSet;
 import static com.google.common.io.Resources.getResource;
 
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
+import com.google.common.collect.ImmutableSet;
+import com.google.common.collect.Sets;
+import com.google.common.flogger.FluentLogger;
 import com.google.common.net.MediaType;
 import com.google.template.soy.SoyFileSet;
 import com.google.template.soy.parseinfo.SoyTemplateInfo;
@@ -40,6 +45,7 @@ import org.joda.time.LocalDate;
 /** Provides e-mail functionality for Spec11 tasks, such as sending Spec11 reports to registrars. */
 public class Spec11EmailUtils {
 
+  private static final FluentLogger logger = FluentLogger.forEnclosingClass();
   private static final SoyTofu SOY_SAUCE =
       SoyFileSet.builder()
           .add(
@@ -81,8 +87,14 @@ public class Spec11EmailUtils {
       SoyTemplateInfo soyTemplateInfo,
       String subject,
       Set<RegistrarThreatMatches> registrarThreatMatchesSet) {
+    // Null/empty email address shouldn't throw an exception, but we should warn if they occur.
+    ImmutableSet<RegistrarThreatMatches> matchesWithoutEmails =
+        registrarThreatMatchesSet.stream()
+            .filter(matches -> isNullOrEmpty(matches.registrarEmailAddress()))
+            .collect(toImmutableSet());
     try {
-      for (RegistrarThreatMatches registrarThreatMatches : registrarThreatMatchesSet) {
+      for (RegistrarThreatMatches registrarThreatMatches :
+          Sets.difference(registrarThreatMatchesSet, matchesWithoutEmails)) {
         emailRegistrar(date, soyTemplateInfo, subject, registrarThreatMatches);
       }
     } catch (Throwable e) {
@@ -92,9 +104,23 @@ public class Spec11EmailUtils {
           String.format("Emailing spec11 reports failed due to %s", getRootCause(e).getMessage()));
       throw new RuntimeException("Emailing spec11 report failed", e);
     }
-    sendAlertEmail(
-        String.format("Spec11 Pipeline Success %s", date),
-        "Spec11 reporting completed successfully.");
+    if (matchesWithoutEmails.isEmpty()) {
+      sendAlertEmail(
+          String.format("Spec11 Pipeline Success %s", date),
+          "Spec11 reporting completed successfully.");
+    } else {
+      logger.atSevere().log(
+          "Some Spec11 threat matches had no associated email addresses: %s", matchesWithoutEmails);
+      // TODO(b/129401965): Use only client IDs in this message
+      sendAlertEmail(
+          String.format("Spec11 Pipeline Warning %s", date),
+          String.format(
+              "No errors occurred but the following matches had no associated email: \n"
+                  + "%s\n\n"
+                  + "This should not occur; please make sure we have email addresses for these"
+                  + " registrar(s).",
+              matchesWithoutEmails));
+    }
   }
 
   private void emailRegistrar(
diff --git a/javatests/google/registry/reporting/spec11/Spec11EmailUtilsTest.java b/javatests/google/registry/reporting/spec11/Spec11EmailUtilsTest.java
index 7a513e748..d9a8e319c 100644
--- a/javatests/google/registry/reporting/spec11/Spec11EmailUtilsTest.java
+++ b/javatests/google/registry/reporting/spec11/Spec11EmailUtilsTest.java
@@ -14,6 +14,7 @@
 
 package google.registry.reporting.spec11;
 
+import static com.google.common.collect.ImmutableSet.toImmutableSet;
 import static com.google.common.truth.Truth.assertThat;
 import static google.registry.reporting.spec11.Spec11RegistrarThreatMatchesParserTest.sampleThreatMatches;
 import static google.registry.testing.JUnitBackports.assertThrows;
@@ -24,6 +25,7 @@ import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
 
 import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableSet;
 import com.google.common.net.MediaType;
 import google.registry.reporting.spec11.soy.Spec11EmailSoyInfo;
 import google.registry.util.EmailMessage;
@@ -221,6 +223,36 @@ public class Spec11EmailUtilsTest {
         Optional.empty());
   }
 
+  @Test
+  public void testWarning_emptyEmailAddressForRegistrars() throws Exception {
+    ImmutableSet<RegistrarThreatMatches> matchesWithoutEmails =
+        sampleThreatMatches().stream()
+            .map(matches -> RegistrarThreatMatches.create("", matches.threatMatches()))
+            .collect(toImmutableSet());
+    emailUtils.emailSpec11Reports(
+        date,
+        Spec11EmailSoyInfo.DAILY_SPEC_11_EMAIL,
+        "Super Cool Registry Daily Threat Detector [2018-07-15]",
+        matchesWithoutEmails);
+    verify(emailService).sendEmail(contentCaptor.capture());
+    validateMessage(
+        contentCaptor.getValue(),
+        "my-sender@test.com",
+        "my-receiver@test.com",
+        Optional.empty(),
+        "Spec11 Pipeline Warning 2018-07-15",
+        "No errors occurred but the following matches had no associated email: \n"
+            + "[RegistrarThreatMatches{registrarEmailAddress=, threatMatches=[ThreatMatch"
+            + "{threatType=MALWARE, platformType=ANY_PLATFORM, metadata=NONE,"
+            + " fullyQualifiedDomainName=a.com}]}, RegistrarThreatMatches{registrarEmailAddress=,"
+            + " threatMatches=[ThreatMatch{threatType=MALWARE, platformType=ANY_PLATFORM,"
+            + " metadata=NONE, fullyQualifiedDomainName=b.com}, ThreatMatch{threatType=MALWARE,"
+            + " platformType=ANY_PLATFORM, metadata=NONE, fullyQualifiedDomainName=c.com}]}]\n\n"
+            + "This should not occur; please make sure we have email addresses for these"
+            + " registrar(s).",
+        Optional.empty());
+  }
+
   private void validateMessage(
       EmailMessage message,
       String from,