diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -1541,6 +1541,7 @@
     'PhabricatorClusterConfigOptions' => 'applications/config/option/PhabricatorClusterConfigOptions.php',
     'PhabricatorCommitBranchesField' => 'applications/repository/customfield/PhabricatorCommitBranchesField.php',
     'PhabricatorCommitCustomField' => 'applications/repository/customfield/PhabricatorCommitCustomField.php',
+    'PhabricatorCommitMergedCommitsField' => 'applications/repository/customfield/PhabricatorCommitMergedCommitsField.php',
     'PhabricatorCommitSearchEngine' => 'applications/audit/query/PhabricatorCommitSearchEngine.php',
     'PhabricatorCommitTagsField' => 'applications/repository/customfield/PhabricatorCommitTagsField.php',
     'PhabricatorCommonPasswords' => 'applications/auth/constants/PhabricatorCommonPasswords.php',
@@ -4906,6 +4907,7 @@
     'PhabricatorClusterConfigOptions' => 'PhabricatorApplicationConfigOptions',
     'PhabricatorCommitBranchesField' => 'PhabricatorCommitCustomField',
     'PhabricatorCommitCustomField' => 'PhabricatorCustomField',
+    'PhabricatorCommitMergedCommitsField' => 'PhabricatorCommitCustomField',
     'PhabricatorCommitSearchEngine' => 'PhabricatorApplicationSearchEngine',
     'PhabricatorCommitTagsField' => 'PhabricatorCommitCustomField',
     'PhabricatorCommonPasswords' => 'Phobject',
diff --git a/src/applications/diffusion/config/PhabricatorDiffusionConfigOptions.php b/src/applications/diffusion/config/PhabricatorDiffusionConfigOptions.php
--- a/src/applications/diffusion/config/PhabricatorDiffusionConfigOptions.php
+++ b/src/applications/diffusion/config/PhabricatorDiffusionConfigOptions.php
@@ -20,6 +20,21 @@
   }
 
   public function getOptions() {
+    $custom_field_type = 'custom:PhabricatorCustomFieldConfigOptionType';
+
+    $fields = array(
+      new PhabricatorCommitBranchesField(),
+      new PhabricatorCommitTagsField(),
+      new PhabricatorCommitMergedCommitsField(),
+    );
+
+    $default_fields = array();
+    foreach ($fields as $field) {
+      $default_fields[$field->getFieldKey()] = array(
+        'disabled' => $field->shouldDisableByDefault(),
+      );
+    }
+
     return array(
       $this->newOption(
         'metamta.diffusion.subject-prefix',
@@ -124,6 +139,13 @@
             'from web traffic (for example, if you use different SSH and '.
             'web load balancers), you can set the SSH hostname here. This '.
             'is an advanced option.')),
+      $this->newOption('diffusion.fields', $custom_field_type, $default_fields)
+        ->setCustomData(
+          id(new PhabricatorRepositoryCommit())
+          ->getCustomFieldBaseClass())
+        ->setDescription(pht(
+          "Select and reorder diffusion fields.\n\n".
+          "These will primarily show up in Mail Notifications.")),
     );
   }
 
diff --git a/src/applications/repository/customfield/PhabricatorCommitMergedCommitsField.php b/src/applications/repository/customfield/PhabricatorCommitMergedCommitsField.php
new file mode 100644
--- /dev/null
+++ b/src/applications/repository/customfield/PhabricatorCommitMergedCommitsField.php
@@ -0,0 +1,66 @@
+<?php
+
+final class PhabricatorCommitMergedCommitsField
+  extends PhabricatorCommitCustomField {
+
+  public function getFieldKey() {
+    return 'diffusion:mergedcommits';
+  }
+
+  public function shouldDisableByDefault() {
+    return true;
+  }
+
+  public function shouldAppearInTransactionMail() {
+    return true;
+  }
+
+  public function updateTransactionMailBody(
+    PhabricatorMetaMTAMailBody $body,
+    PhabricatorApplicationTransactionEditor $editor,
+    array $xactions) {
+
+    // Put all the merged commits info int the mail body if this is a merge
+    $merges_caption = '';
+    // TODO: Make this limit configurable after T6030
+    $limit = 50;
+    $commit = $this->getObject();
+
+    try {
+      $merges = DiffusionPathChange::newFromConduit(
+        id(new ConduitCall('diffusion.mergedcommitsquery', array(
+          'commit' => $commit->getCommitIdentifier(),
+          'limit' => $limit + 1,
+          'callsign' => $commit->getRepository()->getCallsign(),
+        )))
+        ->setUser($this->getViewer())
+        ->execute());
+
+      if (count($merges) > $limit) {
+        $merges = array_slice($merges, 0, $limit);
+        $merges_caption =
+          pht("This commit merges more than %d changes. Only the first ".
+          "%d are shown.\n", $limit, $limit);
+      }
+
+      if ($merges) {
+        $merge_commits = array();
+        foreach ($merges as $merge) {
+          $merge_commits[] = $merge->getAuthorName().
+            ': '.
+            $merge->getSummary();
+        }
+        $body->addTextSection(
+          pht('MERGED COMMITS'),
+          $merges_caption.implode("\n", $merge_commits));
+      }
+    } catch (ConduitException $ex) {
+      // Log the exception into the email body
+      $body->addTextSection(
+        pht('MERGED COMMITS'),
+        pht('Error generating merged commits: ').$ex->getMessage());
+    }
+
+  }
+
+}
diff --git a/src/applications/repository/storage/PhabricatorRepositoryCommit.php b/src/applications/repository/storage/PhabricatorRepositoryCommit.php
--- a/src/applications/repository/storage/PhabricatorRepositoryCommit.php
+++ b/src/applications/repository/storage/PhabricatorRepositoryCommit.php
@@ -349,9 +349,7 @@
 
 
   public function getCustomFieldSpecificationForRole($role) {
-    // TODO: We could make this configurable eventually, but just use the
-    // defaults for now.
-    return array();
+    return PhabricatorEnv::getEnvConfig('diffusion.fields');
   }
 
   public function getCustomFieldBaseClass() {