implement only monitoring dns blocklists, without using them for incoming deliveries

so you can still know when someone has put you on their blocklist (which may
affect delivery), without using them.

also query dnsbls for our ips more often when we do more outgoing connections
for delivery: once every 100 messages, but at least 5 mins and at most 3 hours
since the previous check.

mox-/admin.go

@@ -1069,6 +1069,35 @@ func AccountLimitsSave(ctx context.Context, account string, maxOutgoingMessagesP
 	return nil
 }
 
+func MonitorDNSBLsSave(ctx context.Context, zones []dns.Domain) (rerr error) {
+	log := pkglog.WithContext(ctx)
+	defer func() {
+		if rerr != nil {
+			log.Errorx("saving monitor dnsbl zones", rerr)
+		}
+	}()
+
+	Conf.dynamicMutex.Lock()
+	defer Conf.dynamicMutex.Unlock()
+
+	c := Conf.Dynamic
+
+	// Compose new config without modifying existing data structures. If we fail, we
+	// leave no trace.
+	nc := c
+	nc.MonitorDNSBLs = make([]string, len(zones))
+	nc.MonitorDNSBLZones = nil
+	for i, z := range zones {
+		nc.MonitorDNSBLs[i] = z.Name()
+	}
+
+	if err := writeDynamic(ctx, log, nc); err != nil {
+		return fmt.Errorf("writing domains.conf: %v", err)
+	}
+	log.Info("monitor dnsbl zones saved")
+	return nil
+}
+
 type TLSMode uint8
 
 const (