Backup Mail service using CPanel

We are currently running a server with CPanel on it for our customers, but we were thinking of offering a backup mail service for our customers on ADSL/dialup lines that run their own mail server.

I did a search on google and WHT, and found that I can do it with Exim (http://www.webhostingtalk.com/showth...hreadid=305037)

Now, if we had the following would this work?

Customer has his domain etc hosted on our server, except mail which is sent to his server at home.
If his line went down, and he was using our backup service, would the email still be sent to him once he came back online, of would it get delivered locally?

In other words, would we need to have a seperate server to run a backup service on?

 

 

 

 

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