Thanks, things get clearer but my ISP doesn't seem to be willing to help me. He says I must have a static IP. Here after is their answer :
"It is not possible to redirect mail in this way to our servers - this is
because of the security setup we employ so that our services cannot be abused.
To have a system like this you really need to have a static IP address ,
you could then use our Zone record systems to setup a mail server system."
So where can I find someone to be my secondary mx, is this a service you can find like free dns service,... (probably a stupid question, I guess I 'd better change my ISP)
I know that the problem would be solved with a static IP but I'm connected with isdn router and use dhcp.
Thanks a lot for you help
Jean-Francois