If you want any kind of newsletter functionality for your site, go to Campaign Monitor. Use my computer. Just go.
I’ve built my own newsletter modules in the past, and while it’s interesting and satisfying, you’re just never going to do it as well as guys who spend all day long trying to make emails get through spam filters and display OK in the fifty different email clients in regular use (you thought browser compatibility for web was a challenge? Try email).
There’s both a templated version (you define the template, and get a client to enter their content into predefined fields) or a version where you upload the fully-finished HTML document. It can extract the plain text version for you, manage your subscribers - including a handy Javascript-based subscribe/unsubscribe form - and best of all, add tracking to every link in your email, so you can see reports of who clicked on what, and which bits of your newsletter really got people interested.
The basic pricing is $5USD per mailling plus 1 cent per mail sent, which you can mark up if reselling the service to clients.
