diff --git a/src/wp-includes/PHPMailer/PHPMailer.php b/src/wp-includes/PHPMailer/PHPMailer.php
index 8e51a2a9ba..73c6d2b67b 100644
--- a/src/wp-includes/PHPMailer/PHPMailer.php
+++ b/src/wp-includes/PHPMailer/PHPMailer.php
@@ -750,7 +750,7 @@ class PHPMailer
      *
      * @var string
      */
-    const VERSION = '6.6.5';
+    const VERSION = '6.7';
 
     /**
      * Error severity: message only, continue processing.
@@ -858,7 +858,7 @@ class PHPMailer
     private function mailPassthru($to, $subject, $body, $header, $params)
     {
         //Check overloading of mail function to avoid double-encoding
-        if (ini_get('mbstring.func_overload') & 1) { // phpcs:ignore PHPCompatibility.IniDirectives.RemovedIniDirectives.mbstring_func_overloadDeprecated
+        if ((int)ini_get('mbstring.func_overload') & 1) { // phpcs:ignore PHPCompatibility.IniDirectives.RemovedIniDirectives.mbstring_func_overloadDeprecated
             $subject = $this->secureHeader($subject);
         } else {
             $subject = $this->encodeHeader($this->secureHeader($subject));
@@ -1124,6 +1124,22 @@ class PHPMailer
         return call_user_func_array([$this, 'addAnAddress'], $params);
     }
 
+    /**
+     * Set the boundaries to use for delimiting MIME parts.
+     * If you override this, ensure you set all 3 boundaries to unique values.
+     * The default boundaries include a "=_" sequence which cannot occur in quoted-printable bodies,
+     * as suggested by https://www.rfc-editor.org/rfc/rfc2045#section-6.7
+     *
+     * @return void
+     */
+    public function setBoundaries()
+    {
+        $this->uniqueid = $this->generateId();
+        $this->boundary[1] = 'b1=_' . $this->uniqueid;
+        $this->boundary[2] = 'b2=_' . $this->uniqueid;
+        $this->boundary[3] = 'b3=_' . $this->uniqueid;
+    }
+
     /**
      * Add an address to one of the recipient arrays or to the ReplyTo array.
      * Addresses that have been added already return false, but do not throw exceptions.
@@ -1673,11 +1689,11 @@ class PHPMailer
                     return $this->mailSend($this->MIMEHeader, $this->MIMEBody);
             }
         } catch (Exception $exc) {
+            $this->setError($exc->getMessage());
+            $this->edebug($exc->getMessage());
             if ($this->Mailer === 'smtp' && $this->SMTPKeepAlive == true && $this->smtp->connected()) {
                 $this->smtp->reset();
             }
-            $this->setError($exc->getMessage());
-            $this->edebug($exc->getMessage());
             if ($this->exceptions) {
                 throw $exc;
             }
@@ -2796,10 +2812,7 @@ class PHPMailer
     {
         $body = '';
         //Create unique IDs and preset boundaries
-        $this->uniqueid = $this->generateId();
-        $this->boundary[1] = 'b1_' . $this->uniqueid;
-        $this->boundary[2] = 'b2_' . $this->uniqueid;
-        $this->boundary[3] = 'b3_' . $this->uniqueid;
+        $this->setBoundaries();
 
         if ($this->sign_key_file) {
             $body .= $this->getMailMIME() . static::$LE;
@@ -2835,7 +2848,7 @@ class PHPMailer
             $altBodyEncoding = static::ENCODING_QUOTED_PRINTABLE;
         }
         //Use this as a preamble in all multipart message types
-        $mimepre = 'This is a multi-part message in MIME format.' . static::$LE . static::$LE;
+        $mimepre = '';
         switch ($this->message_type) {
             case 'inline':
                 $body .= $mimepre;
@@ -3071,6 +3084,18 @@ class PHPMailer
         return $body;
     }
 
+    /**
+     * Get the boundaries that this message will use
+     * @return array
+     */
+    public function getBoundaries()
+    {
+        if (empty($this->boundary)) {
+            $this->setBoundaries();
+        }
+        return $this->boundary;
+    }
+
     /**
      * Return the start of a message boundary.
      *
@@ -4188,6 +4213,7 @@ class PHPMailer
      * @param string      $name  Custom header name
      * @param string|null $value Header value
      *
+     * @return bool True if a header was set successfully
      * @throws Exception
      */
     public function addCustomHeader($name, $value = null)
@@ -4637,15 +4663,27 @@ class PHPMailer
     }
 
     /**
-     * Remove trailing breaks from a string.
+     * Remove trailing whitespace from a string.
+     *
+     * @param string $text
+     *
+     * @return string The text to remove whitespace from
+     */
+    public static function stripTrailingWSP($text)
+    {
+        return rtrim($text, " \r\n\t");
+    }
+
+    /**
+     * Strip trailing line breaks from a string.
      *
      * @param string $text
      *
      * @return string The text to remove breaks from
      */
-    public static function stripTrailingWSP($text)
+    public static function stripTrailingBreaks($text)
     {
-        return rtrim($text, " \r\n\t");
+        return rtrim($text, "\r\n");
     }
 
     /**
@@ -4811,7 +4849,7 @@ class PHPMailer
         $body = static::normalizeBreaks($body, self::CRLF);
 
         //Reduce multiple trailing line breaks to a single one
-        return static::stripTrailingWSP($body) . self::CRLF;
+        return static::stripTrailingBreaks($body) . self::CRLF;
     }
 
     /**
diff --git a/src/wp-includes/PHPMailer/SMTP.php b/src/wp-includes/PHPMailer/SMTP.php
index 312f753cea..4d0ac0e540 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 = '6.6.5';
+    const VERSION = '6.7';
 
     /**
      * SMTP line break constant.