I agree with Dynashox, if you are looking for autoresponders Aweber and Getresponse are my recommendations, there are others as well such as IContact and ConstantContact that i know of but the rest I am not familiar with. If you have your own email hosting server to use for your list and you need an mass email sending software, I suggest you try Gammadyne Mailer which I highly recommend
Aweber and GetResponse are among the best (if not the best). The great thing about GetResponse is that it's free for the first 100 subscribers.
I'm currently using mailchimp. I've used constant contact previously but I didn't think the $15 dollars justified the use. Mailchimp on the other hand is free (If you don't send a lot of emails or just getting started). I just found out it has an autoresponder for customers birthdays and will send out a coupon. I'm sure they have other neat options. Try it out, its free.. lol
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mailchip doesn't take affiliate links ... if you want to run your own, you could try phplist as well if not, constantcontact is a popular one
I use bulletproofwebs.com and databasemarketingdepot.com they have there own software and offer unlimited sending. Ive use almost all of those and they cant handle a big load and cost way to much. anyhow just fyi
If you're interested in paying per email blast rather than the size of your list, then check out campaign monitor.
getresponse has a free version... you can start with free and if your business in growing you can change your plans... I like this system comapred to aweber