diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
index 0c4945f6dd..40b384796e 100644
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -430,6 +430,8 @@ phutil_register_library_map(array(
     'ManiphestTransactionSaveController' => 'applications/maniphest/controller/transactionsave',
     'ManiphestTransactionType' => 'applications/maniphest/constants/transactiontype',
     'ManiphestView' => 'applications/maniphest/view/base',
+    'MetaMTAConstants' => 'applications/metamta/constants/base',
+    'MetaMTANotificationType' => 'applications/metamta/constants/notificationtype',
     'Phabricator404Controller' => 'applications/base/controller/404',
     'PhabricatorAuditActionConstants' => 'applications/audit/constants/action',
     'PhabricatorAuditComment' => 'applications/audit/storage/auditcomment',
@@ -1184,6 +1186,7 @@ phutil_register_library_map(array(
     'ManiphestTransactionSaveController' => 'ManiphestController',
     'ManiphestTransactionType' => 'ManiphestConstants',
     'ManiphestView' => 'AphrontView',
+    'MetaMTANotificationType' => 'MetaMTAConstants',
     'Phabricator404Controller' => 'PhabricatorController',
     'PhabricatorAuditComment' => 'PhabricatorAuditDAO',
     'PhabricatorAuditController' => 'PhabricatorController',
diff --git a/src/applications/differential/mail/base/DifferentialMail.php b/src/applications/differential/mail/base/DifferentialMail.php
index 70fcc3bfb4..2a4351a9f0 100644
--- a/src/applications/differential/mail/base/DifferentialMail.php
+++ b/src/applications/differential/mail/base/DifferentialMail.php
@@ -1,7 +1,7 @@
 <?php
 
 /*
- * Copyright 2011 Facebook, Inc.
+ * Copyright 2012 Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -103,6 +103,11 @@ abstract class DifferentialMail {
     $template->setIsBulk(true);
     $template->setRelatedPHID($this->getRevision()->getPHID());
 
+    $mailtags = $this->getMailTags();
+    if ($mailtags) {
+      $template->setMailTags($mailtags);
+    }
+
     $phids = array();
     foreach ($to_phids as $phid) {
       $phids[$phid] = true;
@@ -134,6 +139,10 @@ abstract class DifferentialMail {
     }
   }
 
+  protected function getMailTags() {
+    return array();
+  }
+
   protected function getSubjectPrefix() {
     return PhabricatorEnv::getEnvConfig('metamta.differential.subject-prefix');
   }
diff --git a/src/applications/differential/mail/comment/DifferentialCommentMail.php b/src/applications/differential/mail/comment/DifferentialCommentMail.php
index 39139d662e..397ecd5f2e 100644
--- a/src/applications/differential/mail/comment/DifferentialCommentMail.php
+++ b/src/applications/differential/mail/comment/DifferentialCommentMail.php
@@ -44,6 +44,35 @@ class DifferentialCommentMail extends DifferentialMail {
 
   }
 
+  protected function getMailTags() {
+    $comment = $this->getComment();
+    $action = $comment->getAction();
+
+    $tags = array();
+    switch ($action) {
+      case DifferentialAction::ACTION_ADDCCS:
+        $tags[] = MetaMTANotificationType::TYPE_DIFFERENTIAL_CC;
+        break;
+      case DifferentialAction::ACTION_COMMIT:
+        $tags[] = MetaMTANotificationType::TYPE_DIFFERENTIAL_COMMITTED;
+        break;
+    }
+
+    if (strlen(trim($comment->getContent()))) {
+      switch ($action) {
+        case DifferentialAction::ACTION_COMMIT:
+          // Commit comments are auto-generated and not especially interesting,
+          // so don't tag them as having a comment.
+          break;
+        default:
+          $tags[] = MetaMTANotificationType::TYPE_DIFFERENTIAL_COMMENT;
+          break;
+      }
+    }
+
+    return $tags;
+  }
+
   protected function renderSubject() {
     $verb = ucwords($this->getVerb());
     $revision = $this->getRevision();
diff --git a/src/applications/differential/mail/comment/__init__.php b/src/applications/differential/mail/comment/__init__.php
index b624a2bd37..2dc867eda3 100644
--- a/src/applications/differential/mail/comment/__init__.php
+++ b/src/applications/differential/mail/comment/__init__.php
@@ -11,6 +11,7 @@ phutil_require_module('arcanist', 'differential/constants/revisionstatus');
 phutil_require_module('phabricator', 'applications/differential/constants/action');
 phutil_require_module('phabricator', 'applications/differential/mail/base');
 phutil_require_module('phabricator', 'applications/differential/storage/comment');
+phutil_require_module('phabricator', 'applications/metamta/constants/notificationtype');
 phutil_require_module('phabricator', 'applications/phid/handle/data');
 phutil_require_module('phabricator', 'infrastructure/env');
 
diff --git a/src/applications/maniphest/editor/transaction/ManiphestTransactionEditor.php b/src/applications/maniphest/editor/transaction/ManiphestTransactionEditor.php
index 6ac800617f..21a24d8926 100644
--- a/src/applications/maniphest/editor/transaction/ManiphestTransactionEditor.php
+++ b/src/applications/maniphest/editor/transaction/ManiphestTransactionEditor.php
@@ -1,7 +1,7 @@
 <?php
 
 /*
- * Copyright 2011 Facebook, Inc.
+ * Copyright 2012 Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -253,6 +253,8 @@ class ManiphestTransactionEditor {
     $prefix = $this->getSubjectPrefix();
     $subject = trim("{$prefix} [{$action}] T{$task_id}: {$title}");
 
+    $mailtags = $this->getMailTags($transactions);
+
     $template = id(new PhabricatorMetaMTAMail())
       ->setSubject($subject)
       ->setFrom($transaction->getAuthorPHID())
@@ -261,6 +263,7 @@ class ManiphestTransactionEditor {
       ->setThreadID($thread_id, $is_create)
       ->setRelatedPHID($task->getPHID())
       ->setIsBulk(true)
+      ->setMailTags($mailtags)
       ->setBody($body);
 
     $mails = $reply_handler->multiplexMail(
@@ -349,4 +352,30 @@ class ManiphestTransactionEditor {
     return $is_create;
   }
 
+  private function getMailTags(array $transactions) {
+    $tags = array();
+    foreach ($transactions as $xaction) {
+      switch ($xaction->getTransactionType()) {
+        case ManiphestTransactionType::TYPE_CCS:
+          $tags[] = MetaMTANotificationType::TYPE_MANIPHEST_CC;
+          break;
+        case ManiphestTransactionType::TYPE_PROJECTS:
+          $tags[] = MetaMTANotificationType::TYPE_MANIPHEST_PROJECTS;
+          break;
+        case ManiphestTransactionType::TYPE_PRIORITY:
+          $tags[] = MetaMTANotificationType::TYPE_MANIPHEST_PRIORITY;
+          break;
+        default:
+          $tags[] = MetaMTANotificationType::TYPE_MANIPHEST_OTHER;
+          break;
+      }
+
+      if ($xaction->hasComments()) {
+        $tags[] = MetaMTANotificationType::TYPE_MANIPHEST_COMMENT;
+      }
+    }
+
+    return array_unique($tags);
+  }
+
 }
diff --git a/src/applications/maniphest/editor/transaction/__init__.php b/src/applications/maniphest/editor/transaction/__init__.php
index 09a4a08fb1..1f6fa0b5be 100644
--- a/src/applications/maniphest/editor/transaction/__init__.php
+++ b/src/applications/maniphest/editor/transaction/__init__.php
@@ -12,6 +12,7 @@ phutil_require_module('phabricator', 'applications/maniphest/constants/action');
 phutil_require_module('phabricator', 'applications/maniphest/constants/status');
 phutil_require_module('phabricator', 'applications/maniphest/constants/transactiontype');
 phutil_require_module('phabricator', 'applications/maniphest/view/transactiondetail');
+phutil_require_module('phabricator', 'applications/metamta/constants/notificationtype');
 phutil_require_module('phabricator', 'applications/metamta/storage/mail');
 phutil_require_module('phabricator', 'applications/phid/handle/data');
 phutil_require_module('phabricator', 'applications/search/index/indexer/maniphest');
diff --git a/src/applications/metamta/constants/base/MetaMTAConstants.php b/src/applications/metamta/constants/base/MetaMTAConstants.php
new file mode 100644
index 0000000000..15b55adaba
--- /dev/null
+++ b/src/applications/metamta/constants/base/MetaMTAConstants.php
@@ -0,0 +1,21 @@
+<?php
+
+/*
+ * Copyright 2012 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.
+ */
+
+abstract class MetaMTAConstants {
+
+}
diff --git a/src/applications/metamta/constants/base/__init__.php b/src/applications/metamta/constants/base/__init__.php
new file mode 100644
index 0000000000..0ed1e35a5d
--- /dev/null
+++ b/src/applications/metamta/constants/base/__init__.php
@@ -0,0 +1,10 @@
+<?php
+/**
+ * This file is automatically generated. Lint this module to rebuild it.
+ * @generated
+ */
+
+
+
+
+phutil_require_source('MetaMTAConstants.php');
diff --git a/src/applications/metamta/constants/notificationtype/MetaMTANotificationType.php b/src/applications/metamta/constants/notificationtype/MetaMTANotificationType.php
new file mode 100644
index 0000000000..9200fdf6e9
--- /dev/null
+++ b/src/applications/metamta/constants/notificationtype/MetaMTANotificationType.php
@@ -0,0 +1,32 @@
+<?php
+
+/*
+ * Copyright 2012 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.
+ */
+
+final class MetaMTANotificationType
+  extends MetaMTAConstants {
+
+  const TYPE_DIFFERENTIAL_COMMITTED   = 'differential-committed';
+  const TYPE_DIFFERENTIAL_CC          = 'differential-cc';
+  const TYPE_DIFFERENTIAL_COMMENT     = 'differential-comment';
+
+  const TYPE_MANIPHEST_PROJECTS       = 'maniphest-projects';
+  const TYPE_MANIPHEST_PRIORITY       = 'maniphest-priority';
+  const TYPE_MANIPHEST_CC             = 'maniphest-cc';
+  const TYPE_MANIPHEST_OTHER          = 'maniphest-other';
+  const TYPE_MANIPHEST_COMMENT        = 'maniphest-comment';
+
+}
diff --git a/src/applications/metamta/constants/notificationtype/__init__.php b/src/applications/metamta/constants/notificationtype/__init__.php
new file mode 100644
index 0000000000..8e3dc3cc04
--- /dev/null
+++ b/src/applications/metamta/constants/notificationtype/__init__.php
@@ -0,0 +1,12 @@
+<?php
+/**
+ * This file is automatically generated. Lint this module to rebuild it.
+ * @generated
+ */
+
+
+
+phutil_require_module('phabricator', 'applications/metamta/constants/base');
+
+
+phutil_require_source('MetaMTANotificationType.php');
diff --git a/src/applications/metamta/controller/send/PhabricatorMetaMTASendController.php b/src/applications/metamta/controller/send/PhabricatorMetaMTASendController.php
index 420e2e36d0..81a476a582 100644
--- a/src/applications/metamta/controller/send/PhabricatorMetaMTASendController.php
+++ b/src/applications/metamta/controller/send/PhabricatorMetaMTASendController.php
@@ -1,7 +1,7 @@
 <?php
 
 /*
- * Copyright 2011 Facebook, Inc.
+ * Copyright 2012 Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -46,6 +46,7 @@ class PhabricatorMetaMTASendController extends PhabricatorMetaMTAController {
       $mail->setSimulatedFailureCount($request->getInt('failures'));
       $mail->setIsHTML($request->getInt('html'));
       $mail->setIsBulk($request->getInt('bulk'));
+      $mail->setMailTags($request->getStrList('mailtags'));
       $mail->save();
       if ($request->getInt('immediately')) {
         $mail->sendNow();
@@ -117,6 +118,12 @@ class PhabricatorMetaMTASendController extends PhabricatorMetaMTAController {
         id(new AphrontFormTextAreaControl())
           ->setLabel('Body')
           ->setName('body'))
+      ->appendChild(
+        id(new AphrontFormTextControl())
+          ->setLabel('Mail Tags')
+          ->setName('mailtags')
+          ->setCaption(
+            'Example: <tt>differential-cc, differential-comment</tt>'))
       ->appendChild(
         id(new AphrontFormDragAndDropUploadControl())
           ->setLabel('Attach Files')
diff --git a/src/applications/metamta/storage/mail/PhabricatorMetaMTAMail.php b/src/applications/metamta/storage/mail/PhabricatorMetaMTAMail.php
index aa2bec6211..c85e76ca88 100644
--- a/src/applications/metamta/storage/mail/PhabricatorMetaMTAMail.php
+++ b/src/applications/metamta/storage/mail/PhabricatorMetaMTAMail.php
@@ -63,6 +63,20 @@ class PhabricatorMetaMTAMail extends PhabricatorMetaMTADAO {
     return idx($this->parameters, $param);
   }
 
+  /**
+   * Set tags (@{class:MetaMTANotificationType} constants) which identify the
+   * content of this mail in a general way. These tags are used to allow users
+   * to opt out of receiving certain types of mail, like updates when a task's
+   * projects change.
+   *
+   * @param list<const> List of @{class:MetaMTANotificationType} constants.
+   * @return this
+   */
+  public function setMailTags(array $tags) {
+    $this->setParam('mailtags', $tags);
+    return $this;
+  }
+
   /**
    * In Gmail, conversations will be broken if you reply to a thread and the
    * server sends back a response without referencing your Message-ID, even if
@@ -310,8 +324,8 @@ class PhabricatorMetaMTAMail extends PhabricatorMetaMTADAO {
       $reply_to_name = idx($params, 'reply-to-name', '');
       unset($params['reply-to-name']);
 
-      $add_cc = null;
-      $add_to = null;
+      $add_cc = array();
+      $add_to = array();
 
       foreach ($params as $key => $value) {
         switch ($key) {
@@ -418,6 +432,9 @@ class PhabricatorMetaMTAMail extends PhabricatorMetaMTADAO {
             $thread_index = $this->generateThreadIndex($value, $is_first);
             $mailer->addHeader('Thread-Index', $thread_index);
             break;
+          case 'mailtags':
+            // Handled below.
+            break;
           default:
             // Just discard.
         }
@@ -425,6 +442,60 @@ class PhabricatorMetaMTAMail extends PhabricatorMetaMTADAO {
 
       $mailer->addHeader('X-Mail-Transport-Agent', 'MetaMTA');
 
+
+      // If the message has mailtags, filter out any recipients who don't want
+      // to receive this type of mail.
+      $mailtags = $this->getParam('mailtags');
+      if ($mailtags && ($add_to || $add_cc)) {
+
+        $tag_header = array();
+        foreach ($mailtags as $mailtag) {
+          $tag_header[] = '<'.$mailtag.'>';
+        }
+        $tag_header = implode(', ', $tag_header);
+        $mailer->addHeader('X-Phabricator-Mail-Tags', $tag_header);
+
+        $exclude = array();
+
+        $all_recipients = array_merge(
+          array_keys($add_to),
+          array_keys($add_cc));
+
+        $all_prefs = id(new PhabricatorUserPreferences())->loadAllWhere(
+          'userPHID in (%Ls)',
+          $all_recipients);
+        $all_prefs = mpull($all_prefs, null, 'getUserPHID');
+
+        foreach ($all_recipients as $recipient) {
+          $prefs = idx($all_prefs, $recipient);
+          if (!$prefs) {
+            continue;
+          }
+
+          $user_mailtags = $prefs->getPreference(
+            PhabricatorUserPreferences::PREFERENCE_MAILTAGS,
+            array());
+
+          // The user must have elected to receive mail for at least one
+          // of the mailtags.
+          $send = false;
+          foreach ($mailtags as $tag) {
+            if (idx($user_mailtags, $tag, true)) {
+              $send = true;
+              break;
+            }
+          }
+
+          if (!$send) {
+            $exclude[$recipient] = true;
+          }
+        }
+
+        $add_to = array_diff_key($add_to, $exclude);
+        $add_cc = array_diff_key($add_cc, $exclude);
+      }
+
+
       if ($add_to) {
         $mailer->addTos($add_to);
         if ($add_cc) {
@@ -535,7 +606,7 @@ class PhabricatorMetaMTAMail extends PhabricatorMetaMTADAO {
       if (isset($exclude[$phid])) {
         continue;
       }
-      $emails[] = $handles[$phid]->getEmail();
+      $emails[$phid] = $handles[$phid]->getEmail();
     }
 
     return $emails;
diff --git a/src/applications/people/controller/settings/panels/emailpref/PhabricatorUserEmailPreferenceSettingsPanelController.php b/src/applications/people/controller/settings/panels/emailpref/PhabricatorUserEmailPreferenceSettingsPanelController.php
index 31af4d02f7..b4632d64ab 100644
--- a/src/applications/people/controller/settings/panels/emailpref/PhabricatorUserEmailPreferenceSettingsPanelController.php
+++ b/src/applications/people/controller/settings/panels/emailpref/PhabricatorUserEmailPreferenceSettingsPanelController.php
@@ -43,6 +43,14 @@ class PhabricatorUserEmailPreferenceSettingsPanelController
         $pref_no_self_mail,
         $request->getStr($pref_no_self_mail));
 
+
+      $new_tags = $request->getArr('mailtags');
+      $mailtags = $preferences->getPreference('mailtags', array());
+      foreach ($this->getMailTags() as $key => $label) {
+        $mailtags[$key] = (bool)idx($new_tags, $key, false);
+      }
+      $preferences->setPreference('mailtags', $mailtags);
+
       $preferences->save();
 
       return id(new AphrontRedirectResponse())
@@ -105,10 +113,38 @@ class PhabricatorUserEmailPreferenceSettingsPanelController
             ))
           ->setValue($re_prefix_value));
 
+    $form
+      ->appendChild(
+        '<br />'.
+        '<p class="aphront-form-instructions">'.
+          'You can customize what mail you receive from Phabricator here.'.
+        '</p>'.
+        '<p class="aphront-form-instructions">'.
+          '<strong>NOTE:</strong> If an update makes several changes (like '.
+          'adding CCs to a task, closing it, and adding a comment) you will '.
+          'still receive an email as long as at least one of the changes '.
+          'is set to notify you.'.
+        '</p>'
+        );
+
+    $mailtags = $preferences->getPreference('mailtags', array());
+
+    $form
+      ->appendChild(
+        $this->buildMailTagCheckboxes(
+          $this->getDifferentialMailTags(),
+          $mailtags)
+          ->setLabel('Differential'))
+      ->appendChild(
+        $this->buildMailTagCheckboxes(
+          $this->getManiphestMailTags(),
+          $mailtags)
+          ->setLabel('Maniphest'));
+
     $form
       ->appendChild(
         id(new AphrontFormSubmitControl())
-          ->setValue('Save'));
+          ->setValue('Save Preferences'));
 
     $panel = new AphrontPanelView();
     $panel->setHeader('Email Preferences');
@@ -122,4 +158,55 @@ class PhabricatorUserEmailPreferenceSettingsPanelController
           $panel,
         ));
   }
+
+  private function getMailTags() {
+    return array(
+      MetaMTANotificationType::TYPE_DIFFERENTIAL_CC =>
+        "Send me email when a revision's CCs change.",
+      MetaMTANotificationType::TYPE_DIFFERENTIAL_COMMITTED =>
+        "Send me email when a revision is committed.",
+      MetaMTANotificationType::TYPE_MANIPHEST_PROJECTS =>
+        "Send me email when a task's associated projects change.",
+      MetaMTANotificationType::TYPE_MANIPHEST_PRIORITY =>
+        "Send me email when a task's priority changes.",
+      MetaMTANotificationType::TYPE_MANIPHEST_CC =>
+        "Send me email when a task's CCs change.",
+    );
+  }
+
+  private function getManiphestMailTags() {
+    return array_select_keys(
+      $this->getMailTags(),
+      array(
+        MetaMTANotificationType::TYPE_MANIPHEST_PROJECTS,
+        MetaMTANotificationType::TYPE_MANIPHEST_PRIORITY,
+        MetaMTANotificationType::TYPE_MANIPHEST_CC,
+      ));
+  }
+
+  private function getDifferentialMailTags() {
+    return array_select_keys(
+      $this->getMailTags(),
+      array(
+        MetaMTANotificationType::TYPE_DIFFERENTIAL_CC,
+        MetaMTANotificationType::TYPE_DIFFERENTIAL_COMMITTED,
+      ));
+  }
+
+  private function buildMailTagCheckboxes(
+    array $tags,
+    array $prefs) {
+
+    $control = new AphrontFormCheckboxControl();
+    foreach ($tags as $key => $label) {
+      $control->addCheckbox(
+        'mailtags['.$key.']',
+        1,
+        $label,
+        idx($prefs, $key, 1));
+    }
+
+    return $control;
+  }
+
 }
diff --git a/src/applications/people/controller/settings/panels/emailpref/__init__.php b/src/applications/people/controller/settings/panels/emailpref/__init__.php
index 8a7376d88d..c6bf80fd00 100644
--- a/src/applications/people/controller/settings/panels/emailpref/__init__.php
+++ b/src/applications/people/controller/settings/panels/emailpref/__init__.php
@@ -7,10 +7,12 @@
 
 
 phutil_require_module('phabricator', 'aphront/response/redirect');
+phutil_require_module('phabricator', 'applications/metamta/constants/notificationtype');
 phutil_require_module('phabricator', 'applications/people/controller/settings/panels/base');
 phutil_require_module('phabricator', 'applications/people/storage/preferences');
 phutil_require_module('phabricator', 'infrastructure/env');
 phutil_require_module('phabricator', 'view/form/base');
+phutil_require_module('phabricator', 'view/form/control/checkbox');
 phutil_require_module('phabricator', 'view/form/control/select');
 phutil_require_module('phabricator', 'view/form/control/submit');
 phutil_require_module('phabricator', 'view/form/error');
diff --git a/src/applications/people/storage/preferences/PhabricatorUserPreferences.php b/src/applications/people/storage/preferences/PhabricatorUserPreferences.php
index 193aca39e4..89aa6c9780 100644
--- a/src/applications/people/storage/preferences/PhabricatorUserPreferences.php
+++ b/src/applications/people/storage/preferences/PhabricatorUserPreferences.php
@@ -24,6 +24,7 @@ class PhabricatorUserPreferences extends PhabricatorUserDAO {
 
   const PREFERENCE_RE_PREFIX      = 're-prefix';
   const PREFERENCE_NO_SELF_MAIL   = 'self-mail';
+  const PREFERENCE_MAILTAGS       = 'mailtags';
 
   protected $userPHID;
   protected $preferences = array();