Email marketing's understandably a hot topic today, so we wanted to point you to a terrific resource right here on Sitepoint to answer all the questions about it.
Here it is - The Ultimate Guide to Email Marketing
and here's a detailed list of 13 email service providers you can use to send your emails to your clients and prospects.
Search the forums here if you're looking for info on a particular Email Service Provider, and remember that most of these providers offer free trials, so it's best to start with a list of features you want in a provider then select a small number of services to try yourself to see which you like best.
Another two recent articles: [Rules for Best Practice Email Design: Layout and [URL="http://www.sitepoint.com/rules-best-practice-email-design-content/"]Rules for Best Practice Email Design: Content](http://www.sitepoint.com/best-practice-email/)