I am a Mac user, but I am sure there are others who are Window users. Therefore, I think a system that can easily interface with both Mac and window users' address contacts would be great. Also, it should have a user-friendly button to create "subscribe" and "unsubscribe" feature. Finally, I think it should also have a comprehensive tracking feedback that monitors what our customers are reading; e.g. how long they spend reading (open and delete our mail as opposed to actual reading it); which sub-headers are they reading, etc. Recently Joe Workman debut his "Call to Action" stack for Rapid Weaver https://preview.joeworkman.net/call-to- … src=email. I think a feature like that incorporated to our email would be out of this world!.

I've been a partner with Constant Contact from the beginning but they're behind the times and don't offer responsive emails. I've just started building sites with RSD and created a great email with RED but I am having trouble integrating it into a Constant Contact custom email. Has anyone had any experience with them and RED?

I regularly send to over 10,000 subscribers using emails generated in RED using phplist. Its free and it just takes a little setting up to get it working okay.
I export the html from RED and with a couple of tweaks, just paste it into phplist. (Make sure you use a raw html import option, perhaps switch off the wysiwig editor.)
Then phplist can be configured to limit the number of emails sent per period, so check with your hosting and configure it to fall withing these limits.
I thought i'd mention phplist here as I didn't notice it had a mention yet and some of the commercial options can get quite expensive!