Wordpress action for multipart email with wp_mail function

composer.json

{
  "require": {
    "soundasleep/html2text": "~0.3"
  }
}

functions.php

<?php

require __DIR__ . '/vendor/autoload.php';

add_action( 'phpmailer_init', 'phpmailer_init' );

//http://wordpress.stackexchange.com/a/191974
//http://stackoverflow.com/a/2564472
function phpmailer_init( $phpmailer )
{
  if( $phpmailer->ContentType == 'text/html' ) {
	$phpmailer->AltBody = Html2Text\Html2Text::convert( $phpmailer->Body );
  }
}

Please see this gist: https://gist.github.com/youri--/c4618740b7c50c549314eaebc9f78661
It describes and fixes a huge problem when sending consecutive mails.

A simplified fix follows:

add_filter( 'wp_mail', 'force_phpmailer_reinit_for_multiple_mails', -1 );
function force_phpmailer_reinit_for_multiple_mails( $wp_mail_atts ) {
	global $phpmailer;

	if ( $phpmailer instanceof PHPMailer && $phpmailer->alternativeExists() ) {
		// AltBody property is set, so WordPress must already have used this
		// $phpmailer object just now to send mail, so let's
		// clear the AltBody property
		$phpmailer->AltBody = '';
	}

	// Return untouched atts
	return $wp_mail_atts;
}