I currently use Subscribe by email plugin from WPMUDEV. I also use S3 Offload plugin from Delicious Brains. Everything works smoothly until I select "Remove Local Media" option from the plugin. This means that everything is offloaded to Amazon S3 (and CloudFront) and original media is deleted from the server. When I do this, however, the Subscribe by Email plugin, cannot grab the "Featured Image" and the image is broken when I check the email I received from being a subscriber.
I need this to not link to the local media but link to the Amazon S3 destination. I can probably get the Developers to engage in this thread as well if needed. Below if the URL to the plugin.
Also, here is a page where they talk about compatibility.