Mail-in-a-Box Redundancy Service

[donutbrew]

I agree that a backup box is nice, but OP is worried about deliverability:

@Eliter

In the event that a user’s Mail-in-a-Box server does down, mail would be sent to Mail-in-a-Box Redundancy Service (let’s call it “MIABRS”). MIABRS would hold that mail until their server becomes online again, then send that mail to them and delete the mail. In the event that the server does not come back online again for 48 hours, the mail is deleted.

I’m just advising that this feature is already built in to the mail protocol.