Newsletter Services Better Than WordPress Newsletter Plugins
This is just my opinion. Of course, there might be times when you might want to use a WordPress Newsletter Plugin. But often, if you are planning to run a serious newsletter campaigns and have a lot of subscribers, you probably would be better off to use a Newsletter Service than a WordPress Newsletter Plugin, even though the former costs more and the latter is often free.
The reason is because Newsletter Services such as Constant Contact and Aweber (two of the more popular ones) have been in this kind of dedicated business for years. They got the servers and infrastructure all ironed out and they are aware of and complies to CAN-SPAM policies with proper unsubscribe links. They got pre-made newsletter templates that actually looks nice. They got friendly user-interfaces that enables clients to compose HTML newsletters themselves (without messing the template up).
That can not always be said of WordPress Newsletter Plugins, which has to run on the same host as their wordpress and often sends out newsletter using PHP’s mail command. Sometime clients are on economical shared webhost servers, which may not be built or designed for you to pump out hundreds of newsletters at a time.
The servers of Constant Contact and Aweber are designed especially for this kind of task — pumping out hundreds of newsletters out for hundreds of clients. That is why they cost money. They will give you code snippets to place into your wordpress template. Or you can use a WordPress plugin (such as Magic Action Box) to add a lead generation optin form on your wordpress that integrate into your Constant Contact or Aweber services.