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I want to privately peer-to-peer send emails between me and my friend's self hosted personal email servers. How can I make sure a message from me (A) gets delivered to my friend (B), if me and my friend's servers are never online at the same time, but at least one of our mutual friends (C, D, and E) will be online overlapping at times with each of us, but I don't know which of C, D, and E ahead of time?

Ideally I'd like it to work like this:

  1. My mail server (A) tries to deliver directly to (B).
  2. If I don't receive a successful delivery status notification within some time limit, my mail server sends the email to C, D, and E, as relays for later delivery to B.

What technologies can this be accomplished with? I imagine this is something people would want in messy mesh networks?

keyandthegate
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  • Configure C, D, E as *relays* for your domains (as opposed to being a *final destination* which are A and B) and use [ETRN](https://www.postfix.org/ETRN_README.html) to check mail on C, D and E when either A or B goes online. // No, typical mesh networks don't work like this. They build directed routes in advance, and if the peer is unreachable at the moment of delivery, the delivery fails. Nobody stores packets for deferred delivery. This includes Internet itself, which *is* a mesh network. – Nikita Kipriyanov Jun 29 '22 at 12:26
  • As you combine asynchronous relay and initially unknown optimal route, you get into territory simply not supported for email use cases. Unsupported, because it simply does not produce a more manageable and reliable solution than just doing the obvious thing: [In a business environment](https://serverfault.com/help/on-topic), the obvious solution is to involve a relay that is usually online. – anx Jun 29 '22 at 13:14

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