Postfix on a NixOS null client with external catch-all

I wanted to set up a server so that any local email (e.g. generated by cron jobs/systemd timers) would be forwarded to an external address, regardless of the user. I also wanted the from address to keep the system hostname whilst not allowing any external use of the mailserver.

It took me a while to figure out how to this, so I thought I'd share my method.

Here's the config that can be used to do this on any NixOS host, after redefining the first two variables.

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services.postfix = let
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  localUser = "example-user";
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  forwardingAddress = "user@external.domain";
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in
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{
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  enable = true;
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  destination = [];
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  domain = config.networking.domain;
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  virtual = ''
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    @${config.networking.hostName}.${config.networking.domain} ${localUser}
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    ${localUser} ${forwardingAddress}
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  '';
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  config = {
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    inet_interfaces = "loopback-only";
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  };
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};

Emails to any user without a domain part are all sent to the forwarding address with a clear from address (e.g. System administrator <root@host.example.com>).

Background

First, the basic setup for a null client can be found in the postfix documentation. The example config would be translated into NixOS like so:

services.postfix = {
  enable = true;
  destination = [];
  domain = config.networking.domain;
  origin = config.networking.domain;
  relayHost = config.networking.domain;
  lookupMX = true;
  config = {
    inet_interfaces = "loopback-only";
  };
};

However, this rewrites user@hostname.example.com to user@example.com (due to origin on line 5). I wanted to be able to see which host a mail concerns.