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