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

[jfly]

Split this out from #485 (comment).

My rspamd even classifies your recent mails with ARC_NA.

We SNM uses rspamd, which has support for ARC. Perhaps it needs to be enabled? I haven't investigated.

[mweinelt]

Because it requires setting up a key just like DKIM.