@fernandocortes Are you talking about email autoresponders? or are you talking about these service providers who offer that sort of guarantee things in which to help you send bulk email to the unknown people (they buy the data from other source)?

If it is about autoresponder, I would recommend go for Aweber (yea, my company use it), reasons are:

1. Affordable - price is reasonable, they have different offer that suit with you different needs.
2. Easy to Set Up – They have a step by step instructions on how to set up a list and schedule emails.
3. Unlimited - Aweber let you set up unlimited lists – at no additional charge (not always the case with other autoresponders).
4. Reliable - This autoresponder boast something like 98% deliverability rate.
5. Analyzable – You can track your email, do split testing, see how many subscribers opened your emails or even clicked on the links, AND being able to resend the same email to only those subscribers who haven’t opened it to begin with.
6. Embeddable - design your forms, can have forms for every occasion, embed it into your blog/website anywhere you wish to ..
   

Cheers
Ann

For me, MailChimp.com hands down. I have 3 accounts with them, 2 are free and 1 in paid. My paid list is in excess of 30k and my free ones range from 20-500. 

• They have a great interface.
• Their API makes it incredibly easy for me to integrate them with my existing systems.
• Their free service is feature-rich enough to be everything you need to get started and grow your list. By the time you hit the limits, you should be generating enough revenue to afford their basic level service.
• When there is a problem, they fess up. I got an email from them this morning. Yesterday they had a catastrophic hardware failure and had to restore from backups. It only affected one of my free accounts. Even so, they wrote me because they lost one of my campaigns. Since I was a free account, they couldn’t give me a refund but they put a credit of 1 month on my account for when I decide to become a paying customer. (and I will)

Their API is their biggest asset for me, but I’m a programmer. You may not find it as interesting.

IMHO, etc.=C=

AWeber – I’ve never had a harder time making a customized email IN MY LIFE than I did helping a client in AWeber this week. For clients new to email, I recommend MailChimp because the back end is SO MUCH EASIER to use than everything else. I used it forever before I upgraded to Infusionsoft. I upgraded because of list management – I have quite a few different customer lists…but since starting out, you’ll probably just have one, MailChimp is the best way to go because USING it is the easiest and it produces very attractive emails too. 

http://www.addresstwo.com It does it all plus it’s a small business CRM for only $50/month at the campaign manager level.  All my clients use it.

I use vertical response because I can buy groups of email credits which are good for a year.  No monthly fee and I have all of my opt-in forms on a different list, so it’s easy for me to send an email to the group I need.