[Exim] MX backup using cyrus imapd folders structure...

Hello.

I use Exim 3.35 for mail server. It works with Cyrus imad and deliver mails in
cyrus directory structure (mailDir).
My server is in production and run fine...
I would try to configure a second mail server like MX.backup.
I don't known what to do to backup correctly to the second mail server, and
when the first stand up, to copy the mails arrived in the second to the
first (in the Cyrus imapd folders)...
What for command must i use (for Exim 3.35)?
Do somebody known a documentation or a howto to help me to do that?

Thank's for your help.

Regards,

--
E.Bullier