A few people have asked us how we send you our newsletters - is it Lyris at $7000 or Listserv at $4000? Neither: it's our Smart Mailer extension!
You can offer your clients the ability to send HTML mails based on a template (either on your machine or at a URL), each personalised with the recipient's name. You can add attachments to everyone, or a group of people, there's progress bars while you're sending them - all this in an external ASP class file, so just a couple of lines on your page. Groovy!