Add "Read More" links to RSS email
Do you want newsletter emails to display each post with a featured image and "Read More" below it?
This setting is not controlled by the Featured Images In RSS plugin.
In WordPress, be sure the Settings > Reading option for RSS is set to "Summary". Your feed will then output just the summary with a "Read More" link.
In Mailchimp: The "Read More" can be added within the Mailchimp template itself, as a link back to the post's URL using *|FEEDITEM:URL|*. This is usually included in the default RSS templates.