This is a great plugin, and is a way to avoid using an outside vendor for smaller sites with smaller lists. It also puts the full responsibility of rDNS and SPF on the sender, rather than dealing with an outside vendor (yes, I know this makes life maybe more difficult for newbies, but easier for non-newbies~~)
Ok, I have many years of experience in building very large server-side email newsletter and listserve systems using both opensource and proprietary email engines in both multi-threaded and single-threaded form. I do not expect this plugin to meet those same criteria, but some simple additions and fixes can make this excellent plugin truly stellar as an email subscription and sending control panel. I can tell lots of great thought and care already went into the layout of the control panel -- it's very easy to use.
Here are my initial thoughts after installing it:
1) Can I export members into an XLS or CSV file? I would like to be able to just download those members attached to the e-newsletter plugin (i.e. separate from WP members/users).
2) For the unsubscribe:
a) Can I customize the terms at the bottom? For example, my Wordpress installation is in a non-English language, so I need to customize that text.
b) What about that resulting URL? Maybe best to show a blank page? I am keeping my e-newsletter subscribers away from the WP backend. So perhaps a simple white page with "You are now unsubscribed" in a centered red box would work?
3) For the email headers, best to alter "Mail-From" to use the from address for the newsletter, and not the server default? I tested it twice and I saw the server default -- is that correct?
4) A "fetch URL" feature would be very nice. This is very important. This allows us to fetch HTML from a URL, rather than cut/paste HTML if we have an HTML email template ready to use. If this is implemented, make sure all characters are escaped so that exactly what is fetched is used for the email.
5) It would be good to have a scheduling function. For example, I create a newsletter now, but I want to wait until August 09, 2011, 04:30 to send it. ON top of that, once fetch URL is implemented, you can fetch either upon prepping the newsletter, or upon sending it (i.e. the fetch URL content may change).
6) Default charsets for the emails are important. For Asia charsets we like to use UTF-8, but we also need the ability to change to another charset if necessary. Also changing from 8-bit on emails is important.
7) I need to add a subscribe form to my blog's outside (i.e. theme). I do not use a blog theme that utilizes widgets (it's old-school php/html without many of the newer WP calls), so I can grab either raw HTML to include in my sidebar, or a shortcode. It would be a nice feature to customize a subscribe form, and then give me the ability to include that either as a shortcode or raw HTML.
Many thanks for your hard work~