Mail reverse DNS check, what Jarl adds to named.conf, lists one per line

jarld checks the reverse DNS of the public zones' MX and SPF addresses as the internet sees it, every 6 hours (Exposure page, jarl exposure mail, jarl_mail_reverse_dns_ok). The Server options page, jarl option generated and GET /options/generated list what Jarl writes into named.conf by itself; stats.rate_limit_log turns off the query-errors logging. Option lists of three or more elements are shown one per line.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/metrics.md

@@ -132,6 +132,7 @@ same values.
 | `jarl_certificate_expiry_timestamp_seconds{certificate,source}` | When each certificate Jarl manages expires ([certificates.md](certificates.md)). |
 | `jarl_certificate_failing{certificate,source}` | 1 when obtaining (or reading) the certificate failed the last time, or it has expired. |
 | `jarl_certificate_renewal_failures{certificate}` | Failed attempts to obtain a certificate since its last success. |
+| `jarl_mail_reverse_dns_ok{zone,address}` | 1 when an address that sends or receives a public zone's mail has a PTR record that resolves back to it, at the last check, every 6 hours ([exposure.md](exposure.md#mail)). |
 | `jarl_encrypted_dns_up{kind,address,port}` | 1 when the DoT or DoH listener answered with the right, current certificate at the last check ([encrypted-dns.md](encrypted-dns.md#health)). |
 
 ### Queries (with `stats.enabled`)
@@ -238,6 +239,10 @@ groups:
         for: 20m
         annotations:
           summary: "DNS over {{ $labels.kind }} on {{ $labels.address }} port {{ $labels.port }} isn't working (see the Encrypted DNS page)"
+      - alert: MailReverseDNS
+        expr: jarl_mail_reverse_dns_ok == 0
+        annotations:
+          summary: "{{ $labels.address }} ({{ $labels.zone }} mail) has no reverse DNS that resolves back: receivers may reject its mail"
       - alert: BlockingPausedForLong
         expr: jarl_blocking_paused == 1
         for: 24h