Mail messages do not arrive

[vasacampana]

I set Outbound Mail to "mail"; SendTestMail reports OK, but I do not receive any mail.

Now, I have scheduled a mass-mail job.
First of all, I send the mail to a test address, and it arrives to the recipient.
Then I send the mail as a test to a group of recipients; I get an OK message from CiviMAil, but none of the recipients receives the mail.
Despite of this, I try to run the effective job; again I get an OK message, the CiviMail report says that all the mails have been sent correctly, but again none of the recipients gets the mail.
Please hel me. Thank you.

[joanne]

Please complete your forum profile so that we know which CMS and which version of CiviCRM you are using.

Have you a correctly configured CRON?

http://wiki.civicrm.org/confluence/display/CRMDOC/Managing+Scheduled+Jobs

[vasacampana]

Cron has been configured and gives the following error:

--2014-11-26 21:01:34--  http://heldercamara.it/chcnet/administrator/components/com_civicrm/civicrm/bin/cron.php?name=admin-net&pass=nemessimo24&key=87577494798e52089d0e0f9aaccd5ada
Risoluzione di heldercamara.it... 149.3.144.209
Connessione a heldercamara.it|149.3.144.209|:80... connesso.
HTTP richiesta inviata, in attesa di risposta... 500 Internal Server Error
2014-11-26 21:01:34 ERRORE 500: Internal Server Error.

mod_fcgid: stderr: PHP Fatal error: Cannot redeclare class JApplicationHelper in /var/www/vhosts/heldercamara.it/httpdocs/chcnet/libraries/cms/application/helper.php on line 20