Hardware and performance

Hi guys, im seba, from argentina, i have a serius problem with email servicie in moodle and the cron file. 

I have installed mooodle 1.9.x running with php 5.3.6 and mysql 5.5.9 over CentOS 5, and i have installed Plesk 10 control panel.

We have two moodles installed on the server, one of them, has 300 students and the other 5000 students. We have the same problem in both courses.

Here is the problem:
we use the email service to alert the user when they have new answers or new topic created on the forum. we use the option abstract by day to send email alert.

On the other hand, we dont have any problem when we send email for:
- new user registration
- private massage
- recovering password
- etc.

Definitely, the problem is with the cron execution and the email sending service.

We have tested everything, but the users doesnt receive the email alert even if  we run the cron manually. However,when we do this, the cron says "execution ok" or something similar, but it is not working, the users are not receiving the email alert.

I NEED HELP, PLEASE USE A CLEAR LANGUAGE BECAUSE I SPEAK SPANISH.

THANKS A LOT, Seba.