panel-notifications: findings of the technical and security reviews

Neither review (read-only agents, Opus and Fable, over 972a9f8..400aa76) has
a Critical finding. Both found the same serious one, and more:

- The notification about refused mail was built on what a rejected row says
  of itself. Its domain is the one in MAIL FROM, which the client typed and
  Postfix has not yet held against the login when the milter refuses: an
  application over its limit could have had the panel write a message per
  made-up domain, to another tenant's users among others. And the same
  status is written when an application is refused for its client address,
  which is no limit. Refusals are now counted by the domain the login is
  registered under, only for logins that exist and only while a limit is set
  on the login or its domain; one subject per sending domain. They are read
  through a partial index of their own (migration 0005): the query walked
  the whole send log every minute.
- Account and user e-mails were checked with the DMARC report validator,
  which refuses Gmail and the like - the owner's own kind of address. A new
  validate.AccountEmail accepts them and refuses a local part beginning "-".
- "Said" now means sent: what arises while nobody can be told, or while
  Postfix does not take the message, is told at a later check. Its ending
  with nobody left to tell is simply over.
- Two DNS checks must be 45 minutes apart as well as two: the last check
  before a restart and the first after it were minutes apart.
- The notifier's own mail waiting in the queue, and its bounce coming back
  to postmaster@ this server, are not the queue's subject. A listing cut at
  its size limit or with a line too long to read is no listing; postqueue -j
  gets 30 seconds.
- The tables must name the selector the panel shows, not only have the line.
- An a/mx lookup that did not answer inside the SPF check made the record
  "not authorising"; it is "cannot tell" now, for the pages too.
- A ceiling of 60 messages an hour; a pass stops between domains when the
  process is asked to stop; sendmail gets a WaitDelay; days are rounded as a
  person counts; a failed read of the switch shows it on, not off.

Recorded as accepted in security.md, with a roadmap candidate
(from-header-binding): an application can have a message signed as the
server's own name - or, as always, as another registered domain - by writing
it into the From: header, which nothing holds against the login.

Left as found: a recipient whose copy failed while another's went out is not
tried again; a certificate that lives under 14 days is reported at each
renewal.

On the test server, for this tree: go vet clean, go test ./... 23 packages
ok, make e2e ok (326 s).

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

internal/dnscheck/spf.go

@@ -2,6 +2,7 @@ package dnscheck
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"net"
 	"strings"
@@ -133,7 +134,14 @@ func (c *Checker) evaluateSPF(ctx context.Context, record, domainName string, ip
 				continue
 			}
 			budget--
-			if qualifier == '+' && c.resolvesTo(ctx, target, ips) {
+			match, failed := c.resolvesTo(ctx, target, ips)
+			if failed {
+				// Nothing was learned about this mechanism: "cannot tell",
+				// not "does not authorise".
+				note("a (the lookup did not answer)")
+				continue
+			}
+			if qualifier == '+' && match {
 				return term, unfollowed
 			}
 
@@ -151,7 +159,12 @@ func (c *Checker) evaluateSPF(ctx context.Context, record, domainName string, ip
 				continue
 			}
 			budget--
-			if qualifier == '+' && c.mxResolvesTo(ctx, target, ips, &budget) {
+			match, failed := c.mxResolvesTo(ctx, target, ips, &budget)
+			if failed {
+				note("mx (the lookup did not answer)")
+				continue
+			}
+			if qualifier == '+' && match {
 				return term, unfollowed
 			}
 
@@ -176,39 +189,49 @@ func (c *Checker) evaluateSPF(ctx context.Context, record, domainName string, ip
 	return "", unfollowed
 }
 
+// lookupUnanswered tells a lookup that failed from one that answered "no such
+// name": only the second is a finding.
+func lookupUnanswered(err error) bool {
+	var dnsErr *net.DNSError
+	return err != nil && !(errors.As(err, &dnsErr) && dnsErr.IsNotFound)
+}
+
 // resolvesTo reports whether name resolves to one of the server's addresses.
-func (c *Checker) resolvesTo(ctx context.Context, name string, ips []net.IP) bool {
+// failed is set when the lookup did not answer, which says nothing either way.
+func (c *Checker) resolvesTo(ctx context.Context, name string, ips []net.IP) (match, failed bool) {
 	addrs, err := c.resolver.LookupIPAddr(ctx, name)
 	if err != nil {
-		return false
+		return false, lookupUnanswered(err)
 	}
 	for _, a := range addrs {
 		for _, ip := range ips {
 			if a.IP.Equal(ip) {
-				return true
+				return true, false
 			}
 		}
 	}
-	return false
+	return false, false
 }
 
 // mxResolvesTo reports whether any of name's MX hosts resolves to one of the
 // server's addresses, spending at most the remaining lookup budget.
-func (c *Checker) mxResolvesTo(ctx context.Context, name string, ips []net.IP, budget *int) bool {
+func (c *Checker) mxResolvesTo(ctx context.Context, name string, ips []net.IP, budget *int) (match, failed bool) {
 	mxs, err := c.resolver.LookupMX(ctx, name)
 	if err != nil {
-		return false
+		return false, lookupUnanswered(err)
 	}
 	for _, mx := range mxs {
 		if *budget <= 0 {
-			return false
+			return false, failed
 		}
 		*budget--
-		if c.resolvesTo(ctx, strings.TrimSuffix(mx.Host, "."), ips) {
-			return true
+		m, f := c.resolvesTo(ctx, strings.TrimSuffix(mx.Host, "."), ips)
+		if m {
+			return true, false
 		}
+		failed = failed || f
 	}
-	return false
+	return false, failed
 }
 
 // coversAny reports whether an ip4:/ip6: value — a bare address or a CIDR —