Thx for the quick answer.

I changed PHP to 5.5 without noticeable change. More is not possible.

I had a look at the PHP config:
sendmail_path : /usr/sbin/smtppush -t -i -f info@mysite.be

No idea what it means.

The server log doesn't help. Disabled functions are : phpinfo, system, exec, passthru, proc_open, shell_exec, popen, setlimit, glob, mysql_pconnect, stream_socket_client"

Jean-Michel