Alway text/plain for sending mail

Hi,
Thank you for your plugin! It’s so great. However, the checking content type before merging messages has been wrong on WordPress 4.9.6 (i have not check other versions yet). I have fixed directly on the plugin as below:

in files:
email-header-footer.php

line 269 need to be updated:

$is_pre_need =  false;
$lt_count = substr_count( '<', $email['message']);
$gt_count = substr_count( '>', $email['message']);
if( $lt_count == 0 && $gt_count == 0 ) {
	$is_pre_need =  true;
}

To:

$is_pre_need = ($email['message'] == strip_tags($email['message']) )? true:false;

I hope you can update this officially from plugin.

Thanks,
Jenshan