Question Mail function not initiated

Hello everybody.
I`m having problems with the users of my forum because they aren`t receiving a mail when an answer of their own posts is answered by any user. The users claims that a message comes up when they try to post a forum message which is "Mail function not initiated"reason why they cannot receive the mail when a new answer is published.
My system info,

Joomla version: 1.5.14 Stable
PHP version: 5.2.8
Mail program: PHP mail function
Email: [email protected] (the mail of my site)
Site name: www.operationexcellence.com
Path for Sendmail: /usr/sbin/sendmail
Autenticacion SMTP: No
Security SMTP: None
Port SMTP: 25
User SMTP: empty
Password SMTP: empty
Server SMTP: localhost


My theory:

I`ve testing the mail options and they aren`t working, so far.
In the other hand, I reviewed the users registered and I found that all of them has the option "receiving mail from the system" disabled, however, the administrator has this option active by default.. I think this could be part of the root cause, however, I don`t have any idea about how configuring joomla to get this option active for the users registered....
Please help with this.

Hello,

Why do you haven't filled the fields : User SMTP and Password SMTP ?

You can read this doc : docs.joomla.org/Get_locally_hosted_Jooml...il_functions_to_work

Xillibit, thanks for the reply.
I have to say that SMTP mail function is good, however, it normally goes to SPAM when a mail is sent. I`ll take this into consideration as a plan B.
I got a serious problem when I tried to test some options in the general configuration. By mistake I put a wrong name in the hosting user and my joomla blocked itself.
The message that comes up when I try to acces the page or the admin, is
*************************************************************************************
Forbidden
You don't have permission to access / on this server.

Additionally, a 403 Forbidden error was encountered while trying to use an ErrorDocument to handle the request.

---

Apache Server at www.operationexcellence.com Port 80

*************************************************************************************
I substitued the file configuration with the correct hosting username, however, it doesn`work.
As you can see I have more problems.
What should I do??

Erick