Cannot send mail as reply to a comment

Hi,

I want my blog to send a mail everytime someone leaves a new comment to a given topic, to all the commentators that made a comment on that post (so they are compelled to come back).

First I installed the plugin ‘WP Comment Auto Responder’, that is exactly what I want, but as much as I tried, it does not send mails. The “send test mail” function of this plugin sends an email just fine, so I dunno which can be the problem.

I also tried another plugins, with same results:

http://wordpress.org/extend/plugins/comment-reply-notification/

http://www.ideashower.com/our_solutions/comment-to-email-wordpress-plugin/

Can anyone help me to find a solution? Thanks!