Question
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Can I force Exim to relay mails with unknown recipient to another host?

Scenario
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I want to stage the Exim to Exchange migration in a way that would allow me to move individual accounts between servers and keeping all accounts functional.

My idea is to:

1. configure Exchange to act as Internal Relay, ie. to relay unrecognized recipients to Exim,
2. perform an analogous configuration on Exim and enforce any form of loop protection at either end (for messages that are unknown on both servers),
3. keep Exim as MX records target until the end of staging,
4. gradually create new accounts on Exchange and disable them on Exim at the same time

I'd also welcome any criticism if the idea sounds crappy for any reason.