Outputting the_post_thumbnail inside the_content()

I’m using the post thumbnail feature and the post notification plugin which sends html emails out to people.

It does not however send out what is NOT inside the_content() function call.

I’d dearly love to figure out a bit of coding/plugin/hacking that would allow me to add post thumbnails as normal, and have ‘the_post_thumbnail()’ outputted inside the_content() call

If you look here:

https://www.nativehiphop.net/featured-artists/once-upon-us/

The main picture is a function called in single.php:

<?php the_post_thumbnail('main-post-img'); ?>
<?php the_content(); ?>

I just want to add the_content function call WITHOUT the_post_thumbnail. Can anyone help? Please?