We'll need a little more information on what it is you want to do exactly. I assume you want to set up a double opt-in mailing list to send follow-up messages to your website visitors & subscribers? If so, I would highly recommend going with a service instead of a software - the industry leader being Aweber:
http://sswt.aweber.com
You can then manage an unlimited number of mailing lists and autoresponders from a browser - so you can manage them from any computer, any location.
If you are looking more at 'spamming' - then you do want software over a service. But that is going to be expensive, and a very short-term solution.