setup a backup mail server

Hi,.

We have an Exchange 2010 Sp1 Hub and mailbox roles and FPE 2010 on a server 2008 R2 server and Exchange 2010 edg and TMG and FPE 2010 on a second server 2008 R2. we host the mail of 3 different domains on this exchange server and our customers access thier mails with OWA, OA en POP3.

Now I want to setup an back-up mail server, that in a case of a mail server failure then we dont loss the emails, until we get the exchange server back on line.

Could some one direct me to any document on how to do this?

Thanks

Shahin