Investigate why ARC doesn't seem to be working with nixos.org (umbriel) mailserver

Split this out from https://github.com/NixOS/infra/issues/485#issuecomment-2791387139.

> My rspamd even classifies your recent mails with ARC_NA.

We SNM uses rspamd, which has support for [ARC](https://rspamd.com/doc/modules/arc.html). Perhaps it needs to be enabled? I haven't investigated.