[Exim] Backup MX

Hi everybody,

I am looking for a long time a solution to that:

I have an Server A with EXIM and a low MX (ie MX 10)
I have an Server B with EXIM and a higher MX (ie MX 20)
If Server A is down, it's easy to send the mails to Server B, but the
problem is: How can I do to send then back to server A when Server A will
be up ?

Thank you very much for your replies.

Best regards

Patrick Chevalier

h10@???