well there are alot of application which supports newsletters...
but if your serious in your business...i would recommend you to use a paid service as free service has very much limited features.
http://www.netinfinium.com/nicCorp/product-ecm.asp
i would recommend this solution. It's cheap, fast and reliable supports sms and fax even.
Check it out...or opt to use the trial version to see how it goes...
cheers