mail server failover

hello

I'm trying to setup a failover mail server
in case my main mail server fails, E-Mails would be sent to this one
I got to configure the DNS so that it works (the MX entries)... but my problem is:
how to configure this failover mail server (running EXIM) to just queue E-Mails and then to transfer to the main server when it's back (ie, trying each 2 hours for 5 days)?

thanks

 

 

 

 

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