Use second Sophos UTM at different location as backup spool for incoming email

Hello everyone,

I have got two locations running Sophos UTM 9 (Home edition), both are responsible for the incoming/outgoing emails for a different domain. Which is working fine, as long as the internet is up and running.

The internet connection at location B is not that good at the moment(the ISP is working on it, but it is going to take quite some time), i would like to use the Sophos from location A to act as a backup email spool. And as soon as the Sophos at location B is available again the Sophos from location A should send all the email intended for location B to the Sophos at location B (which location A was holding in the spool or of course in a different way).

I have access to the domain DNS (and of course to both the Sophos setups) so i could add the IP address from Sophos A to the SPF/MX records from the domain controlled by Sophos B. But i am not sure how to get this up and running. For instance how do i tell Sophos A that, email from @domainB.com should be keep in the spool and send to SophosB when it is up and running again. (Instead of just dropping it at the local Exchange server when it comes in)

I know that most email servers (if not all), will try to send an email a couple of times when the receiving end is not available. But still i would like to have a real backup spool.

Anyone who could tell me how to set this up? I hope that my question makes sense, if you have any questions please do not hasitate to ask :)

Thanks in advance!