diff --git a/message_subscribe_email/config/optional/field.field.flagging.email_node.subscribe_flagging.yml b/message_subscribe_email/config/optional/field.field.flagging.email_node.subscribe_flagging.yml
new file mode 100644
index 0000000..c493d4f
--- /dev/null
+++ b/message_subscribe_email/config/optional/field.field.flagging.email_node.subscribe_flagging.yml
@@ -0,0 +1,20 @@
+langcode: en
+status: true
+dependencies:
+  config:
+    - field.storage.flagging.subscribe_flagging
+    - flag.flag.email_node
+id: flagging.email_node.subscribe_flagging
+field_name: subscribe_flagging
+entity_type: flagging
+bundle: email_node
+label: 'Subscription flagging'
+description: 'The subscribe flagging that this is tied to.'
+required: true
+translatable: true
+default_value: {  }
+default_value_callback: ''
+settings:
+  handler: 'default:flagging'
+  handler_settings: {  }
+field_type: entity_reference
diff --git a/message_subscribe_email/config/optional/field.field.flagging.email_og.subscribe_flagging.yml b/message_subscribe_email/config/optional/field.field.flagging.email_og.subscribe_flagging.yml
new file mode 100644
index 0000000..65dc811
--- /dev/null
+++ b/message_subscribe_email/config/optional/field.field.flagging.email_og.subscribe_flagging.yml
@@ -0,0 +1,20 @@
+langcode: en
+status: true
+dependencies:
+  config:
+    - field.storage.flagging.subscribe_flagging
+    - flag.flag.email_og
+id: flagging.email_og.subscribe_flagging
+field_name: subscribe_flagging
+entity_type: flagging
+bundle: email_og
+label: 'Subscription flagging'
+description: 'The subscribe flagging that this is tied to.'
+required: true
+translatable: true
+default_value: {  }
+default_value_callback: ''
+settings:
+  handler: 'default:flagging'
+  handler_settings: {  }
+field_type: entity_reference
diff --git a/message_subscribe_email/config/optional/field.field.flagging.email_term.subscribe_flagging.yml b/message_subscribe_email/config/optional/field.field.flagging.email_term.subscribe_flagging.yml
new file mode 100644
index 0000000..5fb0ec3
--- /dev/null
+++ b/message_subscribe_email/config/optional/field.field.flagging.email_term.subscribe_flagging.yml
@@ -0,0 +1,20 @@
+langcode: en
+status: true
+dependencies:
+  config:
+    - field.storage.flagging.subscribe_flagging
+    - flag.flag.email_term
+id: flagging.email_term.subscribe_flagging
+field_name: subscribe_flagging
+entity_type: flagging
+bundle: email_term
+label: 'Subscription flagging'
+description: 'The subscribe flagging that this is tied to.'
+required: true
+translatable: true
+default_value: {  }
+default_value_callback: ''
+settings:
+  handler: 'default:flagging'
+  handler_settings: {  }
+field_type: entity_reference
diff --git a/message_subscribe_email/config/optional/field.field.flagging.email_user.subscribe_flagging.yml b/message_subscribe_email/config/optional/field.field.flagging.email_user.subscribe_flagging.yml
new file mode 100644
index 0000000..50802d3
--- /dev/null
+++ b/message_subscribe_email/config/optional/field.field.flagging.email_user.subscribe_flagging.yml
@@ -0,0 +1,20 @@
+langcode: en
+status: true
+dependencies:
+  config:
+    - field.storage.flagging.subscribe_flagging
+    - flag.flag.email_user
+id: flagging.email_user.subscribe_flagging
+field_name: subscribe_flagging
+entity_type: flagging
+bundle: email_user
+label: 'Subscription flagging'
+description: 'The subscribe flagging that this is tied to.'
+required: true
+translatable: true
+default_value: {  }
+default_value_callback: ''
+settings:
+  handler: 'default:flagging'
+  handler_settings: {  }
+field_type: entity_reference
diff --git a/message_subscribe_email/config/optional/field.storage.flagging.subscribe_flagging.yml b/message_subscribe_email/config/optional/field.storage.flagging.subscribe_flagging.yml
new file mode 100644
index 0000000..25fedf1
--- /dev/null
+++ b/message_subscribe_email/config/optional/field.storage.flagging.subscribe_flagging.yml
@@ -0,0 +1,18 @@
+langcode: en
+status: true
+dependencies:
+  module:
+    - flag
+id: flagging.subscribe_flagging
+field_name: subscribe_flagging
+entity_type: flagging
+type: entity_reference
+settings:
+  target_type: flagging
+module: core
+locked: true
+cardinality: 1
+translatable: true
+indexes: {  }
+persist_with_no_fields: false
+custom_storage: false
diff --git a/message_subscribe_email/message_subscribe_email.install b/message_subscribe_email/message_subscribe_email.install
index fd0f0b6..7e4b35b 100644
--- a/message_subscribe_email/message_subscribe_email.install
+++ b/message_subscribe_email/message_subscribe_email.install
@@ -5,6 +5,9 @@
  * Install hooks for the message_subscribe_email module.
  */
 
+use Drupal\field\Entity\FieldConfig;
+use Drupal\field\Entity\FieldStorageConfig;
+
 /**
  * Implements hook_install().
  *
@@ -46,3 +49,39 @@ function message_subscribe_email_install() {
     }
   }
 }
+
+/**
+ * Add an entity reference to the original subscribe flagging.
+ */
+function message_subscribe_email_update_8100() {
+  FieldStorageConfig::create([
+    'entity_type' => 'flagging',
+    'field_name' => 'subscribe_flagging',
+    'type' => 'entity_reference',
+    'settings' => [
+      'target_type' => 'flagging',
+    ],
+    'locked' => TRUE,
+  ])->setCardinality(1)->save();
+
+  $emailPrefix = \Drupal::config('message_subscribe_email.settings')->get('flag_prefix') . '_';
+  $flags = \Drupal::entityTypeManager()->getStorage('flag')->loadMultiple();
+  foreach ($flags as $flag) {
+    if (strpos($flag->id(), $emailPrefix) === 0) {
+      FieldConfig::create([
+        'entity_type' => 'flagging',
+        'bundle' => $flag->id(),
+        'field_name' => 'subscribe_flagging',
+        'type' => 'entity_reference',
+        'cardinality' => 1,
+      ])
+        ->setLabel('Subscription flagging')
+        ->setDescription('The subscribe flagging that this is tied to.')
+        ->setRequired(TRUE)
+        ->setSetting('target_type', 'flagging')
+        ->setSetting('handler', 'default')
+        ->setSetting('handler_settings', [])
+        ->save();
+    }
+  }
+}
diff --git a/message_subscribe_email/message_subscribe_email.module b/message_subscribe_email/message_subscribe_email.module
index cf4d275..165316b 100644
--- a/message_subscribe_email/message_subscribe_email.module
+++ b/message_subscribe_email/message_subscribe_email.module
@@ -9,6 +9,7 @@ use Drupal\Core\Access\AccessResult;
 use Drupal\Core\Entity\EntityInterface;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Session\AccountInterface;
+use Drupal\flag\FlaggingInterface;
 use Drupal\flag\FlagInterface;
 
 /**
@@ -79,8 +80,8 @@ function message_subscribe_email_message_subscribe_get_subscribers_alter(array &
       ->condition('flag_id', $flag_ids, 'IN')
       ->condition('uid', array_keys($uids), 'IN')
       // Ensure to grab the correct flagging.
-      ->condition('entity_type', $entity_type)
-      ->condition('entity_id', $entity_ids, 'IN')
+      ->condition('flagged_entity__target_type', $entity_type)
+      ->condition('flagged_entity__target_id', $entity_ids, 'IN')
       ->groupBy('uid')
       ->execute()
       ->fetchAllAssoc('uid');
@@ -128,3 +129,26 @@ function message_subscribe_email_message_subscribe_admin_settings_form_submit(ar
     ->set('flag_prefix', $form_state->getValue('message_subscribe_email_flag_prefix'))
     ->save();
 }
+
+/**
+ * Implements hook_ENTITY_TYPE_presave().
+ */
+function message_subscribe_email_flagging_presave(FlaggingInterface $entity) {
+  $subscribePrefix = \Drupal::config('message_subscribe.settings')->get('flag_prefix') . '_';
+  $emailPrefix = \Drupal::config('message_subscribe_email.settings')->get('flag_prefix') . '_';
+
+  // Automatically set the reference to the subscribe flagging on creation.
+  if ($entity->isNew() && strpos($entity->bundle(), $emailPrefix) === 0) {
+    $storage = \Drupal::entityTypeManager()->getStorage('flagging');
+    $bundle = str_replace($emailPrefix, $subscribePrefix, $entity->bundle());
+    $result = $storage->getQuery()
+      ->condition('flag_id', $bundle)
+      ->condition('flagged_entity__target_id', $entity->getFlaggableId())
+      ->condition('flagged_entity__target_type', $entity->getFlaggableType())
+      ->condition('uid', $entity->getOwnerId())
+      ->execute();
+    if (!empty($result)) {
+      $entity->subscribe_flagging->target_id = reset($result);
+    }
+  }
+}
diff --git a/message_subscribe_email/message_subscribe_email.post_update.php b/message_subscribe_email/message_subscribe_email.post_update.php
new file mode 100644
index 0000000..afa7cb8
--- /dev/null
+++ b/message_subscribe_email/message_subscribe_email.post_update.php
@@ -0,0 +1,72 @@
+<?php
+
+/**
+ * @file
+ * Post update hook impllementations for Message Subscribe Email.
+ */
+
+/**
+ * Populate the new entity reference field.
+ */
+function message_subscribe_email_post_update_flagging_field(&$sandbox) {
+  $subscribePrefix = \Drupal::config('message_subscribe.settings')->get('flag_prefix') . '_';
+  $emailPrefix = \Drupal::config('message_subscribe_email.settings')->get('flag_prefix') . '_';
+  $storage = \Drupal::entityTypeManager()->getStorage('flagging');
+
+  $ids = $storage->getQuery()
+    ->condition('flag_id', $emailPrefix . '%', 'LIKE')
+    ->notExists('subscribe_flagging')
+    ->range(0, 100)
+    ->execute();
+
+  if (!isset($sandbox['current'])) {
+    $sandbox['current'] = 0;
+    $sandbox['max'] = $storage->getQuery()
+      ->condition('flag_id', $emailPrefix . '%', 'LIKE')
+      ->notExists('subscribe_flagging')
+      ->count()
+      ->execute();
+  }
+
+  foreach ($ids as $id) {
+    $sandbox['current']++;
+    $flagging = $storage->load($id);
+    $bundle = str_replace($emailPrefix, $subscribePrefix, $flagging->bundle());
+    $flaggedEntity = $flagging->flagged_entity->entity;
+
+    // Clean up any flaggings where the flagged entity no longer exists.
+    if (empty($flaggedEntity)) {
+      $flagging->delete();
+      $sandbox['deleted']++;
+      continue;
+    }
+
+    // Look up the corresponding subscribe flagging.
+    $result = $storage->getQuery()
+      ->condition('flag_id', $bundle)
+      ->condition('flagged_entity__target_id', $flaggedEntity->id())
+      ->condition('flagged_entity__target_type', $flaggedEntity->getEntityTypeId())
+      ->condition('uid', $flagging->uid->target_id)
+      ->execute();
+
+    // Populate the new field, if it could be found.
+    if (!empty($result)) {
+      $flagging->subscribe_flagging->target_id = reset($result);
+      $flagging->save();
+    }
+    // Otherwise, clean up the rogue email flag. It can't exist without its
+    // subscribe counterpart.
+    else {
+      $flagging->delete();
+      $sandbox['deleted']++;
+    }
+  }
+
+  $sandbox['#finished'] = empty($sandbox['max']) ? 1 : ($sandbox['current'] / $sandbox['max']);
+  if ($sandbox['#finished'] >= 1) {
+    return t('Updated @updated flagging items and deleted @deleted orphaned flagging items.', ['@updated' => $sandbox['max'], '@deleted' => $sandbox['deleted']]);
+  }
+  elseif (function_exists('drush_print')) {
+    drush_print('Completed ' . $sandbox['current'] . ' / ' . $sandbox['max'] . ' (' . number_format($sandbox['current'] / $sandbox['max'] * 100, 2) . '%)');
+  }
+}