Problem sending email after installing postfix with Domain Keys

Hello,

I have installed postfix from the panel and then I have set up spf and domain keys to send email. I can send emails for the domains I have set up.
The problem is with automatic mail sent from cron task, application updates or problem with a backup.

The mail is not sent and I receive this kind of email :
---------------------------------------------------
This is the mail system at host nsXXXXXX.ovh.net.

I'm sorry to have to inform you that your message could not
be delivered to one...

Problem sending email after installing postfix with Domain Keys