Thanks for the feedback! I tried to add the following line to my newsletter/emails/themes/blank/theme.php file, but am getting an error:
<?php echo do_shortcodes('do_widget id=recent-posts-widget-with-thumbnails-3'); ?>
Fatal error: Uncaught Error: Call to undefined function do_shortcodes() in /home1/columbv2/public_html/accc/wptest/wp-content/plugins/newsletter/emails/themes/blank/theme.php
It seems that the function is not available?