Hi Harshad, the answer to your question would be better to understand from the email that I got from Email Subscriber’s Support Team:
“..We have looked into the issue and it seems it due to a conflict between Email Subscribers and WP-Optimize Premium.
WP-Optimize Premium plugin has a lazy loading option for images. When this option is enabled, featured images aren’t visible both in the preview and in the email sent. We aren’t sure why the lazy loading plugin isn’t working as expected as it is just a normal image like other images on the site but we are getting an error in the browser console related to the featured missing image. Refer to attached Pic. Missing-Featured-Image.gif
We have found a quick solution to the issue, the WP-Optimize plugin does allow to exclude images from lazy loading using CSS classes. We have excluded the post image using the class ‘wp-post-image’ in the plugin’s setting and it is working as expected. We have attached some screenshots for your reference.
You can also contact WP-Optimize’s plugin author if they have some input regarding the issue or a better solution from their end. Also let us know if they need anything from our end to fix the issue, we will look into it…”
Thank you, and Happy New Year!