diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
index e13f816c77..d47aa5cfe4 100644
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -488,6 +488,7 @@ phutil_register_library_map(array(
     'PhabricatorMailImplementationTestAdapter' => 'applications/metamta/adapter/test',
     'PhabricatorMailReplyHandler' => 'applications/metamta/replyhandler/base',
     'PhabricatorMarkupEngine' => 'applications/markup/engine',
+    'PhabricatorMetaMTAAttachment' => 'applications/metamta/storage/mail',
     'PhabricatorMetaMTAController' => 'applications/metamta/controller/base',
     'PhabricatorMetaMTADAO' => 'applications/metamta/storage/base',
     'PhabricatorMetaMTADaemon' => 'applications/metamta/daemon/mta',
diff --git a/src/applications/differential/mail/base/DifferentialMail.php b/src/applications/differential/mail/base/DifferentialMail.php
index c298f533c3..b02ee16377 100644
--- a/src/applications/differential/mail/base/DifferentialMail.php
+++ b/src/applications/differential/mail/base/DifferentialMail.php
@@ -90,13 +90,7 @@ abstract class DifferentialMail {
       ->setParentMessageID($this->parentMessageID)
       ->addHeader('Thread-Topic', $this->getRevision()->getTitle());
 
-    foreach ($attachments as $attachment) {
-      $template->addAttachment(
-        $attachment['data'],
-        $attachment['filename'],
-        $attachment['mimetype']
-      );
-    }
+    $template->setAttachments($attachments);
 
     $template->setThreadID(
       $this->getThreadID(),
@@ -120,6 +114,16 @@ abstract class DifferentialMail {
 
     $handles = id(new PhabricatorObjectHandleData($phids))->loadHandles();
 
+    $event = new PhabricatorEvent(
+      PhabricatorEventType::TYPE_DIFFERENTIAL_WILLSENDMAIL,
+      array(
+        'mail' => $template,
+      )
+    );
+    PhabricatorEventEngine::dispatchEvent($event);
+
+    $template = $event->getValue('mail');
+
     $mails = $reply_handler->multiplexMail(
       $template,
       array_select_keys($handles, $to_phids),
@@ -177,14 +181,8 @@ EOTEXT;
 
   /**
    * You can override this method in a subclass and return array of attachments
-   * to be sent with the email.  Each attachment is a dictionary with 'data',
-   * 'filename' and 'mimetype' keys.  For example:
-   *
-   *   array(
-   *     'data' => 'some text',
-   *     'filename' => 'example.txt',
-   *     'mimetype' => 'text/plain'
-   *   );
+   * to be sent with the email.  Each attachment is an instance of
+   * PhabricatorMetaMTAAttachment.
    */
   protected function buildAttachments() {
     return array();
diff --git a/src/applications/differential/mail/base/__init__.php b/src/applications/differential/mail/base/__init__.php
index c69b84db4d..c92e1dfce6 100644
--- a/src/applications/differential/mail/base/__init__.php
+++ b/src/applications/differential/mail/base/__init__.php
@@ -9,6 +9,9 @@
 phutil_require_module('phabricator', 'applications/metamta/storage/mail');
 phutil_require_module('phabricator', 'applications/phid/handle/data');
 phutil_require_module('phabricator', 'infrastructure/env');
+phutil_require_module('phabricator', 'infrastructure/events/constant/type');
+phutil_require_module('phabricator', 'infrastructure/events/engine');
+phutil_require_module('phabricator', 'infrastructure/events/event');
 
 phutil_require_module('phutil', 'utils');
 
diff --git a/src/applications/differential/mail/reviewrequest/DifferentialReviewRequestMail.php b/src/applications/differential/mail/reviewrequest/DifferentialReviewRequestMail.php
index 2a88dcec60..00288ad0be 100644
--- a/src/applications/differential/mail/reviewrequest/DifferentialReviewRequestMail.php
+++ b/src/applications/differential/mail/reviewrequest/DifferentialReviewRequestMail.php
@@ -104,10 +104,10 @@ abstract class DifferentialReviewRequestMail extends DifferentialMail {
       $bundle = ArcanistBundle::newFromChanges($changes);
       $unified_diff = $bundle->toUnifiedDiff();
 
-      $attachments[] = array(
-        'data' => $unified_diff,
-        'filename' => $filename,
-        'mimetype' => 'text/x-patch; charset=utf-8'
+      $attachments[] = new PhabricatorMetaMTAAttachment(
+        $unified_diff,
+        $filename,
+        'text/x-patch; charset=utf-8'
       );
     }
     return $attachments;
diff --git a/src/applications/differential/mail/reviewrequest/__init__.php b/src/applications/differential/mail/reviewrequest/__init__.php
index e47543ab06..0c4c47cb38 100644
--- a/src/applications/differential/mail/reviewrequest/__init__.php
+++ b/src/applications/differential/mail/reviewrequest/__init__.php
@@ -10,6 +10,7 @@ phutil_require_module('arcanist', 'parser/bundle');
 phutil_require_module('arcanist', 'parser/diff/change');
 
 phutil_require_module('phabricator', 'applications/differential/mail/base');
+phutil_require_module('phabricator', 'applications/metamta/storage/mail');
 phutil_require_module('phabricator', 'applications/phid/handle/data');
 phutil_require_module('phabricator', 'infrastructure/env');
 
diff --git a/src/applications/metamta/controller/send/PhabricatorMetaMTASendController.php b/src/applications/metamta/controller/send/PhabricatorMetaMTASendController.php
index 1a6d482e21..420e2e36d0 100644
--- a/src/applications/metamta/controller/send/PhabricatorMetaMTASendController.php
+++ b/src/applications/metamta/controller/send/PhabricatorMetaMTASendController.php
@@ -34,10 +34,11 @@ class PhabricatorMetaMTASendController extends PhabricatorMetaMTAController {
       if ($files) {
         foreach ($files as $phid) {
           $file = id(new PhabricatorFile())->loadOneWhere('phid = %s', $phid);
-          $mail->addAttachment(
+          $mail->addAttachment(new PhabricatorMetaMTAAttachment(
             $file->loadFileData(),
             $file->getName(),
-            $file->getMimeType());
+            $file->getMimeType()
+          ));
         }
       }
 
diff --git a/src/applications/metamta/storage/mail/PhabricatorMetaMTAAttachment.php b/src/applications/metamta/storage/mail/PhabricatorMetaMTAAttachment.php
new file mode 100644
index 0000000000..155e36ff2b
--- /dev/null
+++ b/src/applications/metamta/storage/mail/PhabricatorMetaMTAAttachment.php
@@ -0,0 +1,56 @@
+<?php
+
+/*
+ * Copyright 2011 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+class PhabricatorMetaMTAAttachment {
+  protected $data;
+  protected $filename;
+  protected $mimetype;
+
+  public function __construct($data, $filename, $mimetype) {
+    $this->setData($data);
+    $this->setFileName($filename);
+    $this->setMimeType($mimetype);
+  }
+
+  public function getData() {
+    return $this->data;
+  }
+
+  public function setData($data) {
+    $this->data = $data;
+    return $this;
+  }
+
+  public function getFileName() {
+    return $this->filename;
+  }
+
+  public function setFileName($filename) {
+    $this->filename = $filename;
+    return $this;
+  }
+
+  public function getMimeType() {
+    return $this->mimetype;
+  }
+
+  public function setMimeType($mimetype) {
+    $this->mimetype = $mimetype;
+    return $this;
+  }
+}
diff --git a/src/applications/metamta/storage/mail/PhabricatorMetaMTAMail.php b/src/applications/metamta/storage/mail/PhabricatorMetaMTAMail.php
index 0ffd7017e1..386ac84d7f 100644
--- a/src/applications/metamta/storage/mail/PhabricatorMetaMTAMail.php
+++ b/src/applications/metamta/storage/mail/PhabricatorMetaMTAMail.php
@@ -104,12 +104,17 @@ class PhabricatorMetaMTAMail extends PhabricatorMetaMTADAO {
     return $this;
   }
 
-  public function addAttachment($data, $filename, $mimetype) {
-    $this->parameters['attachments'][] = array(
-      'data' => $data,
-      'filename' => $filename,
-      'mimetype' => $mimetype
-    );
+  public function addAttachment(PhabricatorMetaMTAAttachment $attachment) {
+    $this->parameters['attachments'][] = $attachment;
+    return $this;
+  }
+
+  public function getAttachments() {
+    return $this->getParam('attachments');
+  }
+
+  public function setAttachments(array $attachments) {
+    $this->setParam('attachments', $attachments);
     return $this;
   }
 
@@ -312,9 +317,9 @@ class PhabricatorMetaMTAMail extends PhabricatorMetaMTADAO {
           case 'attachments':
             foreach ($value as $attachment) {
               $mailer->addAttachment(
-                $attachment['data'],
-                $attachment['filename'],
-                $attachment['mimetype']
+                $attachment->getData(),
+                $attachment->getFileName(),
+                $attachment->getMimeType()
               );
             }
             break;
diff --git a/src/applications/metamta/storage/mail/__init__.php b/src/applications/metamta/storage/mail/__init__.php
index 8e7330cba4..a61fe4f106 100644
--- a/src/applications/metamta/storage/mail/__init__.php
+++ b/src/applications/metamta/storage/mail/__init__.php
@@ -14,4 +14,5 @@ phutil_require_module('phutil', 'symbols');
 phutil_require_module('phutil', 'utils');
 
 
+phutil_require_source('PhabricatorMetaMTAAttachment.php');
 phutil_require_source('PhabricatorMetaMTAMail.php');
diff --git a/src/docs/userguide/events.diviner b/src/docs/userguide/events.diviner
index 2507594abe..c9894c8a29 100644
--- a/src/docs/userguide/events.diviner
+++ b/src/docs/userguide/events.diviner
@@ -24,4 +24,11 @@ or alter the edit. Data available on this event:
     @{class:ManiphestTransaction}) being applied.
   - ##new## A boolean indicating if this task is being created.
   - ##mail## If this edit originates from email, the
-    @{class:PhabricatorMetaMTAReceivedMail} object.
\ No newline at end of file
+    @{class:PhabricatorMetaMTAReceivedMail} object.
+
+== PhabricatorEventType::TYPE_DIFFERENTIAL_WILLSENDMAIL ==
+
+This event is dispatche before Differential sends an email, and allows you to
+edit the message that will be sent.  Data available on this event:
+
+  - ##mail## The {@class:PhabricatorMetaMTAMail} being edited.
diff --git a/src/infrastructure/events/constant/type/PhabricatorEventType.php b/src/infrastructure/events/constant/type/PhabricatorEventType.php
index 4d89a573fe..2822d4f382 100644
--- a/src/infrastructure/events/constant/type/PhabricatorEventType.php
+++ b/src/infrastructure/events/constant/type/PhabricatorEventType.php
@@ -20,5 +20,6 @@ final class PhabricatorEventType extends PhabricatorEventConstants {
 
   const TYPE_ALL                    = '*';
   const TYPE_MANIPHEST_WILLEDITTASK = 'maniphest.willEditTask';
+  const TYPE_DIFFERENTIAL_WILLSENDMAIL = 'differential.willSendMail';
 
 }