Googling my issue doesn't give any useful result so I submit this issue with attached the solution (as a screenshot). I hope it is useful.

The ISSUE in the suject is reproducible if:

1. You use an external SMTP server to send Email reports
2. Your external SMTP server requires fully-qualified-hostname EHLO/HELO from client to grant sending

The SMTP transport class included in the Piwik sources hardcodes the hostname to "localhost" instead of the true local hostname as required by RFC2821, RFC1869. This way the SMTP server refuses to send the report email.

The "Zend_Mail_Transport_Smtp" constructor provides a way to set custom configuration (such as the hostname) but the Piwik interface isn't able to set this variable.

Here I attach my solution :)