Disable implicit mulitplexing in PHPMailerLite

Summary:
PHPMailerLite implicitly multiplexes mail, but we multiplex mail in an application-aware way higher in the stack. Disable the multiplexing.

The actual option is here: https://secure.phabricator.com/diffusion/P/browse/master/externals/phpmailer/class.phpmailer-lite.php;03dafec74f97bcd7$166

Test Plan: @klimek confirms this fixes his issue.

Reviewers: klimek, vrana, btrahan

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D3365

src/applications/metamta/adapter/PhabricatorMailImplementationPHPMailerLiteAdapter.php

@@ -31,6 +31,11 @@ class PhabricatorMailImplementationPHPMailerLiteAdapter
     require_once $root.'/externals/phpmailer/class.phpmailer-lite.php';
     $this->mailer = new PHPMailerLite($use_exceptions = true);
     $this->mailer->CharSet = 'utf-8';
+
+    // By default, PHPMailerLite sends one mail per recipient. We handle
+    // multiplexing higher in the stack, so tell it to send mail exactly
+    // like we ask.
+    $this->mailer->SingleTo = false;
   }
 
   public function supportsMessageIDHeader() {