NIP-05 verification "result: Unknown"

[mdavids]

I see quite a few failed nip-05 verifications in the syslog of my nostr-rs-relay v0.8.9 that I can't explain, including my own.
But many others work fine.

At first I thought it was because of the underscore I used (_@spongat.nl), but changing that (into marco@spongat.nl) did not fix the issue:

Jun 15 07:38:25 nostr docker[978]: #033[2mJun 15 07:38:25.946#033[0m #033[32m INFO#033[0m nostr_rs_relay::nip05: checked name "marco@spongat.nl", result: Unknown, in: 36.221219ms

It's not just mine, there are others as well. When checking them, their nostr.json settings seem okay. And Nostr-clients also indicate they are validated.

[verified_users]
# NIP-05 verification of users.  Can be "enabled" to require NIP-05
# metadata for event authors, "passive" to perform validation but
# never block publishing, or "disabled" to do nothing.
mode = "enabled"

(no black- or white lists configured)