Can the eNewsletter plugin be installed site by site?

I was really hoping to give my clients the ability to use eNewsletter if they signed up and paid for it. So far, I've only seen an option to Network Activate it. I've gone through the video and documentation and have not been able to figure out how to enable it on a site by site basis so that my clients can import their list(s) and begin using their own private eNewsletter system.

If this is not possible, can you recommend another plugin like eNewsletter that can be installed independently for each site?