diff --git a/application/common/components/Emailer.php b/application/common/components/Emailer.php
index f45a4011..4c0a8acb 100644
--- a/application/common/components/Emailer.php
+++ b/application/common/components/Emailer.php
@@ -484,6 +484,26 @@ public function shouldSendAbandonedUsersMessage(): bool
         return !$haveSentAbandonedUsersEmailRecently;
     }
 
+    /**
+     * Whether we should send an external-groups sync-errors email to the given
+     * email address.
+     *
+     * @param string $emailAddress
+     * @return bool
+     */
+    public function shouldSendExternalGroupsSyncErrorsEmailTo(string $emailAddress): bool
+    {
+        if (empty($emailAddress)) {
+            return false;
+        }
+
+        $haveSentEmailRecently = $this->hasNonUserReceivedMessageRecently(
+            $emailAddress,
+            EmailLog::MESSAGE_TYPE_EXT_GROUP_SYNC_ERRORS
+        );
+
+        return !$haveSentEmailRecently;
+    }
 
     /**
      * Whether we should send an invite message to the given User.
@@ -819,28 +839,38 @@ public function sendExternalGroupSyncErrorsEmail(
     ) {
         $logData = [
             'action' => 'send external-groups sync errors email',
-            'status' => 'starting',
+            'prefix' => $appPrefix,
         ];
 
-        $this->logger->info(array_merge($logData, [
-            'errors' => count($errors)
-        ]));
-
-        $this->sendMessageTo(
-            EmailLog::MESSAGE_TYPE_EXT_GROUP_SYNC_ERRORS,
-            null,
-            [
-                'toAddress' => $recipient,
-                'appPrefix' => $appPrefix,
-                'errors' => $errors,
-                'googleSheetUrl' => $googleSheetUrl,
-                'idpDisplayName' => \Yii::$app->params['idpDisplayName'],
-            ]
-        );
+        if (!$this->shouldSendExternalGroupsSyncErrorsEmailTo($recipient)) {
+            $this->logger->info(array_merge($logData, [
+                'errors' => count($errors),
+                'recipient' => $recipient,
+                'status' => 'skipping (too soon to resend)',
+            ]));
+        } else {
+            $this->logger->info(array_merge($logData, [
+                'errors' => count($errors),
+                'recipient' => $recipient,
+                'status' => 'starting',
+            ]));
+
+            $this->sendMessageTo(
+                EmailLog::MESSAGE_TYPE_EXT_GROUP_SYNC_ERRORS,
+                null,
+                [
+                    'toAddress' => $recipient,
+                    'appPrefix' => $appPrefix,
+                    'errors' => $errors,
+                    'googleSheetUrl' => $googleSheetUrl,
+                    'idpDisplayName' => \Yii::$app->params['idpDisplayName'],
+                ]
+            );
 
-        $this->logger->info(array_merge($logData, [
-            'status' => 'finished',
-        ]));
+            $this->logger->info(array_merge($logData, [
+                'status' => 'finished',
+            ]));
+        }
     }
 
     /**
diff --git a/application/common/models/EmailLog.php b/application/common/models/EmailLog.php
index 9b8a98cc..39feb630 100644
--- a/application/common/models/EmailLog.php
+++ b/application/common/models/EmailLog.php
@@ -118,6 +118,53 @@ public function rules()
                 'default',
                 'value' => MySqlDateTime::now(),
             ],
+            [
+                'non_user_address',
+                'emptyIfUserIsSet',
+            ],
+            [
+                'non_user_address',
+                'requiredIfUserIsEmpty',
+                'skipOnEmpty' => false,
+            ],
         ], parent::rules());
     }
+
+    /**
+     * Validate an attribute to ensure it is empty (unset) if `user_id` is set.
+     *
+     * @param string $attribute Attribute name to be validated
+     * @return bool
+     */
+    public function emptyIfUserIsSet($attribute)
+    {
+        if (!empty($this->user_id) && !empty($this[$attribute])) {
+            $this->addError(
+                $attribute,
+                sprintf(
+                    'Only a user_id or a %s may be set, not both',
+                    $attribute
+                )
+            );
+        }
+    }
+
+    /**
+     * Validate an attribute to ensure it is not empty if `user_id` is not set.
+     *
+     * @param string $attribute Attribute name to be validated
+     * @return bool
+     */
+    public function requiredIfUserIsEmpty($attribute)
+    {
+        if (empty($this->user_id) && empty($this[$attribute])) {
+            $this->addError(
+                $attribute,
+                sprintf(
+                    'Either a user_id or a %s must be set',
+                    $attribute
+                )
+            );
+        }
+    }
 }
diff --git a/application/features/bootstrap/EmailContext.php b/application/features/bootstrap/EmailContext.php
index 262c28b3..9f8c3897 100644
--- a/application/features/bootstrap/EmailContext.php
+++ b/application/features/bootstrap/EmailContext.php
@@ -11,7 +11,6 @@
 use common\models\MfaBackupcode;
 use common\models\MfaWebauthn;
 use common\models\User;
-use Sil\SilIdBroker\Behat\Context\YiiContext;
 use Webmozart\Assert\Assert;
 
 class EmailContext extends YiiContext
@@ -59,6 +58,10 @@ class EmailContext extends YiiContext
     /** @var array<array> */
     protected $matchingFakeEmails;
 
+    private string $dummyExtGroupsAppPrefix = 'ext-dummy';
+
+    private ?EmailLog $tempEmailLog;
+
     public const METHOD_EMAIL_ADDRESS = 'method@example.com';
     public const MANAGER_EMAIL = 'manager@example.com';
 
@@ -1243,7 +1246,7 @@ public function theUserHasNotLoggedInFor($months)
     }
 
     /**
-     * @When I send abandoned user email
+     * @When I send abandoned user email (again)
      */
     public function iSendAbandonedUserEmail()
     {
@@ -1292,4 +1295,76 @@ public function theDatabaseHasBeenPurged()
         User::deleteAll();
         $this->fakeEmailer->forgetFakeEmailsSent();
     }
+
+    /**
+     * @Then the abandoned user email has been sent :expectedCount time(s)
+     */
+    public function theAbandonedUserEmailHasBeenSentTime($expectedCount)
+    {
+        $actualCount = $this->countEmailsSent(EmailLog::MESSAGE_TYPE_ABANDONED_USERS);
+        Assert::eq($actualCount, $expectedCount);
+    }
+
+    /**
+     * @Given I send an external-groups sync-error email (again)
+     */
+    public function iSendAnExternalGroupsSyncErrorEmail()
+    {
+        $this->fakeEmailer->sendExternalGroupSyncErrorsEmail(
+            $this->dummyExtGroupsAppPrefix,
+            ['dummy error'],
+            'dummy@example.com',
+            ''
+        );
+    }
+
+    /**
+     * @Then the external-groups sync-error email has been sent :expectedCount time(s)
+     */
+    public function theExternalGroupsSyncErrorEmailHasBeenSentTime($expectedCount)
+    {
+        // The $appPrefix is needed by the FakeEmailer, to find the appropriate
+        // emails (by being able to accurately generate the expected subject).
+        $this->fakeEmailer->otherDataForEmails['appPrefix'] = $this->dummyExtGroupsAppPrefix;
+
+        $actualCount = $this->countEmailsSent(
+            EmailLog::MESSAGE_TYPE_EXT_GROUP_SYNC_ERRORS
+        );
+        Assert::eq($actualCount, $expectedCount);
+    }
+
+    /**
+     * @When I try to log an email as sent to that User and to a non-user address
+     */
+    public function iTryToLogAnEmailAsSentToThatUserAndToANonUserAddress()
+    {
+        $this->tempEmailLog = new EmailLog([
+            'message_type' => EmailLog::MESSAGE_TYPE_ABANDONED_USERS,
+            'non_user_address' => 'dummy@example.com',
+            'user_id' => $this->tempUser->id,
+        ]);
+        $this->tempEmailLog->save();
+    }
+
+    /**
+     * @Then an email log validation error should have occurred
+     */
+    public function anEmailLogValidationErrorShouldHaveOccurred()
+    {
+        Assert::notEmpty(
+            $this->tempEmailLog->errors,
+            'No EmailLog validation errors were found'
+        );
+    }
+
+    /**
+     * @When I try to log an email as sent to neither a User nor a non-user address
+     */
+    public function iTryToLogAnEmailAsSentToNeitherAUserNorANonUserAddress()
+    {
+        $this->tempEmailLog = new EmailLog([
+            'message_type' => EmailLog::MESSAGE_TYPE_ABANDONED_USERS,
+        ]);
+        $this->tempEmailLog->save();
+    }
 }
diff --git a/application/features/email.feature b/application/features/email.feature
index b6d21892..d385669a 100644
--- a/application/features/email.feature
+++ b/application/features/email.feature
@@ -404,7 +404,6 @@ Feature: Email
       | to send      | -1     | has          | should NOT     |
       | NOT to send  | -1     | has NOT      | should NOT     |
 
-
   Scenario Outline: HR notification users
     Given hr notification email <isOrIsNot> set
       And the database has been purged
@@ -423,3 +422,27 @@ Feature: Email
       | is        | an inactive | 7 months  | has NOT     |
       | is        | a           | 5 months  | has NOT     |
       | is        | a           | 7 months  | has         |
+
+  Scenario: Not sending HR notification emails too frequently
+    Given hr notification email is set
+      And the database has been purged
+      And a user already exists
+      And the user has not logged in for "7 months"
+      And I send abandoned user email
+    When I send abandoned user email again
+    Then the abandoned user email has been sent 1 time
+
+  Scenario: Not sending external-group sync-error emails too frequently
+    Given the database has been purged
+      And I send an external-groups sync-error email
+    When I send an external-groups sync-error email again
+    Then the external-groups sync-error email has been sent 1 time
+
+  Scenario: Ensure no EmailLog is to both a User and a non-user address
+    Given a user already exists
+    When I try to log an email as sent to that User and to a non-user address
+    Then an email log validation error should have occurred
+
+  Scenario: Ensure each EmailLog is to either a User or a non-user address
+    When I try to log an email as sent to neither a User nor a non-user address
+    Then an email log validation error should have occurred