diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
index f4eb97a090..9528a958a1 100644
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -1069,6 +1069,7 @@ phutil_register_library_map(array(
     'ManiphestUpdateConduitAPIMethod' => 'applications/maniphest/conduit/ManiphestUpdateConduitAPIMethod.php',
     'ManiphestView' => 'applications/maniphest/view/ManiphestView.php',
     'MetaMTAConstants' => 'applications/metamta/constants/MetaMTAConstants.php',
+    'MetaMTAEmailTransactionCommand' => 'applications/metamta/command/MetaMTAEmailTransactionCommand.php',
     'MetaMTAMailReceivedGarbageCollector' => 'applications/metamta/garbagecollector/MetaMTAMailReceivedGarbageCollector.php',
     'MetaMTAMailSentGarbageCollector' => 'applications/metamta/garbagecollector/MetaMTAMailSentGarbageCollector.php',
     'MetaMTANotificationType' => 'applications/metamta/constants/MetaMTANotificationType.php',
@@ -2544,6 +2545,7 @@ phutil_register_library_map(array(
     'PhabricatorSubscriptionsListController' => 'applications/subscriptions/controller/PhabricatorSubscriptionsListController.php',
     'PhabricatorSubscriptionsTransactionController' => 'applications/subscriptions/controller/PhabricatorSubscriptionsTransactionController.php',
     'PhabricatorSubscriptionsUIEventListener' => 'applications/subscriptions/events/PhabricatorSubscriptionsUIEventListener.php',
+    'PhabricatorSubscriptionsUnsubscribeEmailCommand' => 'applications/subscriptions/command/PhabricatorSubscriptionsUnsubscribeEmailCommand.php',
     'PhabricatorSupportApplication' => 'applications/support/application/PhabricatorSupportApplication.php',
     'PhabricatorSyntaxHighlighter' => 'infrastructure/markup/PhabricatorSyntaxHighlighter.php',
     'PhabricatorSyntaxHighlightingConfigOptions' => 'applications/config/option/PhabricatorSyntaxHighlightingConfigOptions.php',
@@ -4320,6 +4322,7 @@ phutil_register_library_map(array(
     'ManiphestTransactionSaveController' => 'ManiphestController',
     'ManiphestUpdateConduitAPIMethod' => 'ManiphestConduitAPIMethod',
     'ManiphestView' => 'AphrontView',
+    'MetaMTAEmailTransactionCommand' => 'Phobject',
     'MetaMTAMailReceivedGarbageCollector' => 'PhabricatorGarbageCollector',
     'MetaMTAMailSentGarbageCollector' => 'PhabricatorGarbageCollector',
     'MetaMTANotificationType' => 'MetaMTAConstants',
@@ -5921,6 +5924,7 @@ phutil_register_library_map(array(
     'PhabricatorSubscriptionsListController' => 'PhabricatorController',
     'PhabricatorSubscriptionsTransactionController' => 'PhabricatorController',
     'PhabricatorSubscriptionsUIEventListener' => 'PhabricatorEventListener',
+    'PhabricatorSubscriptionsUnsubscribeEmailCommand' => 'MetaMTAEmailTransactionCommand',
     'PhabricatorSupportApplication' => 'PhabricatorApplication',
     'PhabricatorSyntaxHighlightingConfigOptions' => 'PhabricatorApplicationConfigOptions',
     'PhabricatorSystemActionEngine' => 'Phobject',
diff --git a/src/applications/files/mail/FileReplyHandler.php b/src/applications/files/mail/FileReplyHandler.php
index 881bf06519..b21c487e14 100644
--- a/src/applications/files/mail/FileReplyHandler.php
+++ b/src/applications/files/mail/FileReplyHandler.php
@@ -13,22 +13,4 @@ final class FileReplyHandler
     return 'F';
   }
 
-  protected function processMailCommands(array $commands) {
-   $actor = $this->getActor();
-
-    $xactions = array();
-    foreach ($commands as $command) {
-      switch (head($command)) {
-        case 'unsubscribe':
-          $xaction = id(new PhabricatorFileTransaction())
-            ->setTransactionType(PhabricatorTransactions::TYPE_SUBSCRIBERS)
-            ->setNewValue(array('-' => array($actor->getPHID())));
-          $xactions[] = $xaction;
-          break;
-      }
-    }
-
-    return $xactions;
-  }
-
 }
diff --git a/src/applications/metamta/command/MetaMTAEmailTransactionCommand.php b/src/applications/metamta/command/MetaMTAEmailTransactionCommand.php
new file mode 100644
index 0000000000..9e20ef43e1
--- /dev/null
+++ b/src/applications/metamta/command/MetaMTAEmailTransactionCommand.php
@@ -0,0 +1,81 @@
+<?php
+
+abstract class MetaMTAEmailTransactionCommand extends Phobject {
+
+  abstract public function getCommand();
+  abstract public function isCommandSupportedForObject(
+    PhabricatorApplicationTransactionInterface $object);
+  abstract public function buildTransactions(
+    PhabricatorUser $viewer,
+    PhabricatorApplicationTransactionInterface $object,
+    PhabricatorMetaMTAReceivedMail $mail,
+    $command,
+    array $argv);
+
+  public function getCommandAliases() {
+    return array();
+  }
+
+  public function getCommandObjects() {
+    return array($this);
+  }
+
+  public static function getAllCommands() {
+    static $commands;
+
+    if ($commands === null) {
+      $kinds = id(new PhutilSymbolLoader())
+        ->setAncestorClass(__CLASS__)
+        ->loadObjects();
+      $commands = array();
+      foreach ($kinds as $kind) {
+        foreach ($kind->getCommandObjects() as $command) {
+          $commands[] = $command;
+        }
+      }
+    }
+
+    return $commands;
+  }
+
+  public static function getAllCommandsForObject(
+    PhabricatorApplicationTransactionInterface $object) {
+
+    $commands = self::getAllCommands();
+    foreach ($commands as $key => $command) {
+      if (!$command->isCommandSupportedForObject($object)) {
+        unset($commands[$key]);
+      }
+    }
+
+    return $commands;
+  }
+
+  public static function getCommandMap(array $commands) {
+    assert_instances_of($commands, 'MetaMTAEmailTransactionCommand');
+
+    $map = array();
+    foreach ($commands as $command) {
+      $keywords = $command->getCommandAliases();
+      $keywords[] = $command->getCommand();
+
+      foreach ($keywords as $keyword) {
+        $keyword = phutil_utf8_strtolower($keyword);
+        if (empty($map[$keyword])) {
+          $map[$keyword] = $command;
+        } else {
+          throw new Exception(
+            pht(
+              'Mail commands "%s" and "%s" both respond to keyword "%s". '.
+              'Keywords must be uniquely associated with commands.',
+              get_class($command),
+              get_class($map[$keyword]),
+              $keyword));
+        }
+      }
+    }
+
+    return $map;
+  }
+
+}
diff --git a/src/applications/paste/mail/PasteReplyHandler.php b/src/applications/paste/mail/PasteReplyHandler.php
index 64945c5101..b8c498b76e 100644
--- a/src/applications/paste/mail/PasteReplyHandler.php
+++ b/src/applications/paste/mail/PasteReplyHandler.php
@@ -13,22 +13,4 @@ final class PasteReplyHandler
     return 'P';
   }
 
-  protected function processMailCommands(array $commands) {
-   $actor = $this->getActor();
-
-    $xactions = array();
-    foreach ($commands as $command) {
-      switch (head($command)) {
-        case 'unsubscribe':
-          $xaction = id(new PhabricatorPasteTransaction())
-            ->setTransactionType(PhabricatorTransactions::TYPE_SUBSCRIBERS)
-            ->setNewValue(array('-' => array($actor->getPHID())));
-          $xactions[] = $xaction;
-          break;
-      }
-    }
-
-    return $xactions;
-  }
-
 }
diff --git a/src/applications/subscriptions/command/PhabricatorSubscriptionsUnsubscribeEmailCommand.php b/src/applications/subscriptions/command/PhabricatorSubscriptionsUnsubscribeEmailCommand.php
new file mode 100644
index 0000000000..124ba9605a
--- /dev/null
+++ b/src/applications/subscriptions/command/PhabricatorSubscriptionsUnsubscribeEmailCommand.php
@@ -0,0 +1,33 @@
+<?php
+
+final class PhabricatorSubscriptionsUnsubscribeEmailCommand
+  extends MetaMTAEmailTransactionCommand {
+
+  public function getCommand() {
+    return 'unsubscribe';
+  }
+
+  public function isCommandSupportedForObject(
+    PhabricatorApplicationTransactionInterface $object) {
+    return ($object instanceof PhabricatorSubscribableInterface);
+  }
+
+  public function buildTransactions(
+    PhabricatorUser $viewer,
+    PhabricatorApplicationTransactionInterface $object,
+    PhabricatorMetaMTAReceivedMail $mail,
+    $command,
+    array $argv) {
+    $xactions = array();
+
+    $xactions[] = $object->getApplicationTransactionTemplate()
+      ->setTransactionType(PhabricatorTransactions::TYPE_SUBSCRIBERS)
+      ->setNewValue(
+        array(
+          '-' => array($viewer->getPHID()),
+        ));
+
+    return $xactions;
+  }
+
+}
diff --git a/src/applications/transactions/replyhandler/PhabricatorApplicationTransactionReplyHandler.php b/src/applications/transactions/replyhandler/PhabricatorApplicationTransactionReplyHandler.php
index fbfeb6303f..2eb8955f91 100644
--- a/src/applications/transactions/replyhandler/PhabricatorApplicationTransactionReplyHandler.php
+++ b/src/applications/transactions/replyhandler/PhabricatorApplicationTransactionReplyHandler.php
@@ -49,7 +49,9 @@ abstract class PhabricatorApplicationTransactionReplyHandler
 
     $body_data = $mail->parseBody();
 
-    $xactions = $this->processMailCommands($body_data['commands']);
+    $xactions = $this->processMailCommands(
+      $mail,
+      $body_data['commands']);
 
     // If this object is subscribable, subscribe all the users who were
     // CC'd on the message.
@@ -84,9 +86,60 @@ abstract class PhabricatorApplicationTransactionReplyHandler
       ->applyTransactions($target, $xactions);
   }
 
-  protected function processMailCommands(array $commands) {
-    // TODO: Modularize this.
-    return array();
+  private function processMailCommands(
+    PhabricatorMetaMTAReceivedMail $mail,
+    array $command_list) {
+
+    $viewer = $this->getActor();
+    $object = $this->getMailReceiver();
+
+    $list = MetaMTAEmailTransactionCommand::getAllCommandsForObject($object);
+    $map = MetaMTAEmailTransactionCommand::getCommandMap($list);
+
+    $xactions = array();
+    foreach ($command_list as $command_argv) {
+      $command = head($command_argv);
+      $argv = array_slice($command_argv, 1);
+
+      $handler = idx($map, phutil_utf8_strtolower($command));
+      if ($handler) {
+        $results = $handler->buildTransactions(
+          $viewer,
+          $object,
+          $mail,
+          $command,
+          $argv);
+        foreach ($results as $result) {
+          $xactions[] = $result;
+        }
+      } else {
+        $valid_commands = array();
+        foreach ($list as $valid_command) {
+          $aliases = $valid_command->getCommandAliases();
+          if ($aliases) {
+            foreach ($aliases as $key => $alias) {
+              $aliases[$key] = '!'.$alias;
+            }
+            $aliases = implode(', ', $aliases);
+            $valid_commands[] = pht(
+              '!%s (or %s)',
+              $valid_command->getCommand(),
+              $aliases);
+          } else {
+            $valid_commands[] = '!'.$valid_command->getCommand();
+          }
+        }
+
+        throw new Exception(
+          pht(
+            'The command "!%s" is not a supported mail command. Valid '.
+            'commands for this object are: %s.',
+            $command,
+            implode(', ', $valid_commands)));
+      }
+    }
+
+    return $xactions;
   }
 
 }