diff --git a/src/applications/metamta/adapter/PhabricatorMailImplementationTestAdapter.php b/src/applications/metamta/adapter/PhabricatorMailImplementationTestAdapter.php
--- a/src/applications/metamta/adapter/PhabricatorMailImplementationTestAdapter.php
+++ b/src/applications/metamta/adapter/PhabricatorMailImplementationTestAdapter.php
@@ -123,4 +123,12 @@
     return $this;
   }
 
+  public function getBody() {
+    return idx($this->guts, 'body');
+  }
+
+  public function getHTMLBody() {
+    return idx($this->guts, 'html-body');
+  }
+
 }
diff --git a/src/applications/metamta/storage/PhabricatorMetaMTAMail.php b/src/applications/metamta/storage/PhabricatorMetaMTAMail.php
--- a/src/applications/metamta/storage/PhabricatorMetaMTAMail.php
+++ b/src/applications/metamta/storage/PhabricatorMetaMTAMail.php
@@ -892,15 +892,16 @@
       $body = $raw_body;
     }
 
-    $max = PhabricatorEnv::getEnvConfig('metamta.email-body-limit');
-    if (strlen($body) > $max) {
+    $body_limit = PhabricatorEnv::getEnvConfig('metamta.email-body-limit');
+    if (strlen($body) > $body_limit) {
       $body = id(new PhutilUTF8StringTruncator())
-        ->setMaximumBytes($max)
+        ->setMaximumBytes($body_limit)
         ->truncateString($body);
       $body .= "\n";
-      $body .= pht('(This email was truncated at %d bytes.)', $max);
+      $body .= pht('(This email was truncated at %d bytes.)', $body_limit);
     }
     $mailer->setBody($body);
+    $body_limit -= strlen($body);
 
     // If we sent a different message body than we were asked to, record
     // what we actually sent to make debugging and diagnostics easier.
@@ -914,8 +915,16 @@
       $send_html = $this->shouldSendHTML($preferences);
     }
 
-    if ($send_html && isset($params['html-body'])) {
-      $mailer->setHTMLBody($params['html-body']);
+    if ($send_html) {
+      $html_body = idx($params, 'html-body');
+
+      // NOTE: We just drop the entire HTML body if it won't fit. Safely
+      // truncating HTML is hard, and we already have the text body to fall
+      // back to.
+      if (strlen($html_body) <= $body_limit) {
+        $mailer->setHTMLBody($html_body);
+        $body_limit -= strlen($html_body);
+      }
     }
 
     // Pass the headers to the mailer, then save the state so we can show
diff --git a/src/applications/metamta/storage/__tests__/PhabricatorMetaMTAMailTestCase.php b/src/applications/metamta/storage/__tests__/PhabricatorMetaMTAMailTestCase.php
--- a/src/applications/metamta/storage/__tests__/PhabricatorMetaMTAMailTestCase.php
+++ b/src/applications/metamta/storage/__tests__/PhabricatorMetaMTAMailTestCase.php
@@ -331,4 +331,84 @@
     $this->assertEqual(null, $mail->getMailerKey());
   }
 
+  public function testMailSizeLimits() {
+    $env = PhabricatorEnv::beginScopedEnv();
+    $env->overrideEnvConfig('metamta.email-body-limit', 1024 * 512);
+
+    $user = $this->generateNewTestUser();
+    $phid = $user->getPHID();
+
+    $string_1kb = str_repeat('x', 1024);
+    $html_1kb = str_repeat('y', 1024);
+    $string_1mb = str_repeat('x', 1024 * 1024);
+    $html_1mb = str_repeat('y', 1024 * 1024);
+
+    // First, send a mail with a small text body and a small HTML body to make
+    // sure the basics work properly.
+    $mail = id(new PhabricatorMetaMTAMail())
+      ->addTos(array($phid))
+      ->setBody($string_1kb)
+      ->setHTMLBody($html_1kb);
+
+    $mailer = new PhabricatorMailImplementationTestAdapter();
+    $mail->sendWithMailers(array($mailer));
+    $this->assertEqual(
+      PhabricatorMailOutboundStatus::STATUS_SENT,
+      $mail->getStatus());
+
+    $text_body = $mailer->getBody();
+    $html_body = $mailer->getHTMLBody();
+
+    $this->assertEqual($string_1kb, $text_body);
+    $this->assertEqual($html_1kb, $html_body);
+
+
+    // Now, send a mail with a large text body and a large HTML body. We expect
+    // the text body to be truncated and the HTML body to be dropped.
+    $mail = id(new PhabricatorMetaMTAMail())
+      ->addTos(array($phid))
+      ->setBody($string_1mb)
+      ->setHTMLBody($html_1mb);
+
+    $mailer = new PhabricatorMailImplementationTestAdapter();
+    $mail->sendWithMailers(array($mailer));
+    $this->assertEqual(
+      PhabricatorMailOutboundStatus::STATUS_SENT,
+      $mail->getStatus());
+
+    $text_body = $mailer->getBody();
+    $html_body = $mailer->getHTMLBody();
+
+    // We expect the body was truncated, because it exceeded the body limit.
+    $this->assertTrue(
+      (strlen($text_body) < strlen($string_1mb)),
+      pht('Text Body Truncated'));
+
+    // We expect the HTML body was dropped completely after the text body was
+    // truncated.
+    $this->assertTrue(
+      !strlen($html_body),
+      pht('HTML Body Removed'));
+
+
+    // Next send a mail with a small text body and a large HTML body. We expect
+    // the text body to be intact and the HTML body to be dropped.
+    $mail = id(new PhabricatorMetaMTAMail())
+      ->addTos(array($phid))
+      ->setBody($string_1kb)
+      ->setHTMLBody($html_1mb);
+
+    $mailer = new PhabricatorMailImplementationTestAdapter();
+    $mail->sendWithMailers(array($mailer));
+    $this->assertEqual(
+      PhabricatorMailOutboundStatus::STATUS_SENT,
+      $mail->getStatus());
+
+    $text_body = $mailer->getBody();
+    $html_body = $mailer->getHTMLBody();
+
+    $this->assertEqual($string_1kb, $text_body);
+    $this->assertTrue(!strlen($html_body));
+  }
+
 }