diff --git a/src/wp-includes/PHPMailer/PHPMailer.php b/src/wp-includes/PHPMailer/PHPMailer.php
index 0f23b041e2..3a15081170 100644
--- a/src/wp-includes/PHPMailer/PHPMailer.php
+++ b/src/wp-includes/PHPMailer/PHPMailer.php
@@ -350,8 +350,8 @@ class PHPMailer
     public $Password = '';
 
     /**
-     * SMTP auth type.
-     * Options are CRAM-MD5, LOGIN, PLAIN, XOAUTH2, attempted in that order if not specified.
+     * SMTP authentication type. Options are CRAM-MD5, LOGIN, PLAIN, XOAUTH2.
+     * If not specified, the first one from that list that the server supports will be selected.
      *
      * @var string
      */
@@ -750,7 +750,7 @@ class PHPMailer
      *
      * @var string
      */
-    const VERSION = '6.6.3';
+    const VERSION = '6.6.4';
 
     /**
      * Error severity: message only, continue processing.
@@ -1096,7 +1096,7 @@ class PHPMailer
 
             return false;
         }
-        if ($name !== null) {
+        if ($name !== null && is_string($name)) {
             $name = trim(preg_replace('/[\r\n]+/', '', $name)); //Strip breaks and trim
         } else {
             $name = '';
@@ -1288,7 +1288,7 @@ class PHPMailer
      */
     public function setFrom($address, $name = '', $auto = true)
     {
-        $address = trim($address);
+        $address = trim((string)$address);
         $name = trim(preg_replace('/[\r\n]+/', '', $name)); //Strip breaks and trim
         //Don't validate now addresses with IDN. Will be done in send().
         $pos = strrpos($address, '@');
@@ -1893,7 +1893,14 @@ class PHPMailer
         foreach ($this->to as $toaddr) {
             $toArr[] = $this->addrFormat($toaddr);
         }
-        $to = implode(', ', $toArr);
+        $to = trim(implode(', ', $toArr));
+
+        //If there are no To-addresses (e.g. when sending only to BCC-addresses)
+        //the following should be added to get a correct DKIM-signature.
+        //Compare with $this->preSend()
+        if ($to === '') {
+            $to = 'undisclosed-recipients:;';
+        }
 
         $params = null;
         //This sets the SMTP envelope sender which gets turned into a return-path header by the receiver
@@ -4472,6 +4479,7 @@ class PHPMailer
             'ics' => 'text/calendar',
             'xml' => 'text/xml',
             'xsl' => 'text/xml',
+            'csv' => 'text/csv',
             'wmv' => 'video/x-ms-wmv',
             'mpeg' => 'video/mpeg',
             'mpe' => 'video/mpeg',
diff --git a/src/wp-includes/PHPMailer/SMTP.php b/src/wp-includes/PHPMailer/SMTP.php
index c439a888ba..cc1711d2e2 100644
--- a/src/wp-includes/PHPMailer/SMTP.php
+++ b/src/wp-includes/PHPMailer/SMTP.php
@@ -35,7 +35,7 @@ class SMTP
      *
      * @var string
      */
-    const VERSION = '6.6.3';
+    const VERSION = '6.6.4';
 
     /**
      * SMTP line break constant.