How to set up a bunch of "internal mail server + external caching"?

Question

Email

There is a wish to configure the strange scheme of work of mail:
There is a corporate e-mail server in a corporate network.
I would like to set up an external mail server on a vps hosting with few resources so that users continue to work with it when the main server is unavailable (the light was suddenly turned off or the admin on the switch just closed two ports in a loop).
That's it, so that this external server can send and receive new letters, without access to old letters, and when the connection with the main mail server is restored, it must merge all the mail to it and remain "clean" again!
How to make it so that users connect to an external server only when the main one is unavailable is clear in principle (through the priority of MX records), but how to force an external server not to accumulate letters is not clear. Does this happen at all or are my tastes too specific?

Answer 1

What is a "caching" mail server? I also understand the frontend for antispam, antivirus to unload the backend. And so:
Set up two servers of equal value, figure out how and what will roam in the filer mode, test - profit.
Or buy MS Exchange - two servers in different DCs in one DAG - profit.