diff --git a/modules/betterbird/README.md b/modules/betterbird/README.md
index 4e2a2acd..3b814646 100644
--- a/modules/betterbird/README.md
+++ b/modules/betterbird/README.md
@@ -97,7 +97,7 @@ start; `prefs.js` wins over a default.
 value true, false, a whole number or a string. The node-engine merges the layers key by key, so a node's layer
 overrides one preference, or takes one out with `null`. The operator's setup approved on 2026-10-12 (compact
 density, 14-point text, the vertical view with multi-line cards and threads, no new-mail pop-up, sound or tray
-icon, the calendar's day from 09:00 to 20:00, reminders missed while asleep shown, chat off, remote content
+icon, the calendar's day from 09:00 to 20:00, reminders missed while asleep shown (since locked off: issue 640), chat off, remote content
 blocked, Do Not Track; 21 preferences) moved from the catalogue's defaults file into the mesh-wide layer of
 `prefs`, set on the operator's Approve.
 
@@ -109,10 +109,24 @@ desktop node writes the map's text into the old build's user.js, and `betterbird
 refused until the push.
 
 **Locked** (the policy): Betterbird's updater off (`DisableAppUpdate`); telemetry off (`DisableTelemetry`);
-add-ons change only through the mesh's pins (`ExtensionUpdate` false); and three preferences locked off:
+add-ons change only through the mesh's pins (`ExtensionUpdate` false); and four preferences locked off:
 `mail.shell.checkDefaultClient` (the ask to be the default client), `mail.inappnotifications.enabled` (the
-vendor's in-app notices), and `mailnews.start_page.enabled` (the start page, which otherwise loads from the
-vendor at every start).
+vendor's in-app notices), `mailnews.start_page.enabled` (the start page, which otherwise loads from the
+vendor at every start), and `calendar.alarms.showmissed` (the reminders of meetings that have passed, below).
+
+**A passed meeting's reminder is not shown again** (novox/hq issue 640). With `calendar.alarms.showmissed` on,
+Betterbird's alarm service fires, at every load of a calendar (each start, each refresh, and each time Owl
+registers its calendar after signing in), every reminder of the past month whose item carries no later
+acknowledgement (`X-MOZ-LASTACK`). Off, Betterbird's own default, a past reminder fires only while its meeting
+has not ended. A dismissal is that acknowledgement, and Owl never sends it to Exchange (its changes carry the
+reminder's offset alone): it lives only in Betterbird's local calendar cache, which Owl empties when it
+resynchronises a calendar from scratch. With the pass on, Exchange meetings of the past month came back about
+every ten minutes on g14 from the start that placed the setting's `true`, Dismiss or not. So the policy locks the
+pass off, whatever the setting `prefs` or a profile says; a setting's value for it has no effect (`betterbird_config`
+shows the lock). What it costs: a reminder that fell due while the laptop slept is not shown once the meeting
+is over; one for a meeting still running is. `betterbird_check` says when the running Betterbird still has the
+pass on beside an Owl calendar, with the remedy (the module's push, or `betterbird_restart` once the policy is
+placed).
 
 **Kept at Betterbird's default** (`// keeps Betterbird's default: <name>` in `mesh-defaults.js`): the theme,
 system colours, always-underlined links, manual spam marking, and the composer's attachment pane. A value the
diff --git a/modules/betterbird/cmd/betterbird-tools/betterbird.go b/modules/betterbird/cmd/betterbird-tools/betterbird.go
index a76148e3..900d3c5d 100644
--- a/modules/betterbird/cmd/betterbird-tools/betterbird.go
+++ b/modules/betterbird/cmd/betterbird-tools/betterbird.go
@@ -981,6 +981,9 @@ func (m *Machine) Check() (CheckAnswer, error) {
 				"take them out of install/defaults-pref/mesh-defaults.js in a reviewed catalogue change: an account is declared in the setting accounts")
 		}
 	}
+	if what, do := m.missedReminders(profilePath); what != "" {
+		add(what, do)
+	}
 	if why := m.defaultClientAsk(profilePath); why != "" && in.From != "not installed" {
 		add("Betterbird asks at its start to be the default mail client: "+why,
 			"push the module to this node: its policy locks "+defaultClientPref+" off (novox/hq issue 345)")
diff --git a/modules/betterbird/cmd/betterbird-tools/config_test.go b/modules/betterbird/cmd/betterbird-tools/config_test.go
index c92a7288..da612459 100644
--- a/modules/betterbird/cmd/betterbird-tools/config_test.go
+++ b/modules/betterbird/cmd/betterbird-tools/config_test.go
@@ -249,8 +249,8 @@ func TestTheConfigShowsTheLayersAndTheDrift(t *testing.T) {
 	f.withDefaults()
 	f.betterbirdsOwn()
 	links(t, nil)
-	f.declare(`user_pref("mail.uifontsize", 16); user_pref("mail.biff.show_alert", false); user_pref("calendar.alarms.showmissed", true);`)
-	f.write(profileDir+"/user.js", mark+")\n"+`user_pref("mail.uifontsize", 16);`+"\n"+`user_pref("calendar.alarms.showmissed", true);`+"\n")
+	f.declare(`user_pref("mail.uifontsize", 16); user_pref("mail.biff.show_alert", false); user_pref("calendar.alarms.playsound", false);`)
+	f.write(profileDir+"/user.js", mark+")\n"+`user_pref("mail.uifontsize", 16);`+"\n"+`user_pref("calendar.alarms.playsound", false);`+"\n")
 	f.write(profileDir+"/prefs.js", "// Mozilla User Preferences\n"+
 		`user_pref("extensions.activeThemeID", "thunderbird-compact-dark@mozilla.org");`+"\n"+
 		`user_pref("mail.uifontsize", 12);`+"\n"+
@@ -266,7 +266,7 @@ func TestTheConfigShowsTheLayersAndTheDrift(t *testing.T) {
 	for _, p := range c.Prefs {
 		by[p.Name] = p
 	}
-	if len(c.Prefs) != 3+3+len(shippedKeepsWant) || !strings.HasPrefix(c.Defaults, "placed") {
+	if len(c.Prefs) != len(lockedOff)+3+len(shippedKeepsWant) || !strings.HasPrefix(c.Defaults, "placed") {
 		t.Fatalf("every lock, declared preference and kept default: %d, %s", len(c.Prefs), c.Defaults)
 	}
 	if !reflect.DeepEqual(c.Drifting, []string{"extensions.activeThemeID", "mail.spam.manualMark", "mail.biff.show_alert"}) {
@@ -293,7 +293,7 @@ func TestTheConfigShowsTheLayersAndTheDrift(t *testing.T) {
 	if p := by["mailnews.start_page.enabled"]; p.Layer != "locked" || p.Effective != "false" || p.Drift != "" {
 		t.Errorf("a lock wins over the profile: %+v", p)
 	}
-	if p := by["calendar.alarms.showmissed"]; p.Effective != "true" || p.Layer != "declared" {
+	if p := by["calendar.alarms.playsound"]; p.Effective != "false" || p.Layer != "declared" {
 		t.Errorf("%+v", p)
 	}
 	// T5: the check says the drift, with both remedies.
diff --git a/modules/betterbird/cmd/betterbird-tools/install.go b/modules/betterbird/cmd/betterbird-tools/install.go
index e9826e07..ba71bc0a 100644
--- a/modules/betterbird/cmd/betterbird-tools/install.go
+++ b/modules/betterbird/cmd/betterbird-tools/install.go
@@ -18,6 +18,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"os"
 	"path/filepath"
 	"strings"
 	"time"
@@ -172,8 +173,8 @@ func (m *Machine) policyState() string {
 		return "placed, and " + why
 	}
 	return "placed: Betterbird's updater, its telemetry and add-on updates off, Owl installed into every profile from " +
-		"the mesh's xpi with its updates off, and the ask to be the default mail client, the vendor's in-app notices " +
-		"and the start page locked off"
+		"the mesh's xpi with its updates off, and the ask to be the default mail client, the vendor's in-app notices, " +
+		"the start page and the reminders of meetings that have passed locked off"
 }
 
 // lockedOff are the preferences the module's policy locks to false (novox/hq issue 345), each with what it is.
@@ -181,6 +182,70 @@ var lockedOff = []struct{ Name, What string }{
 	{defaultClientPref, "the ask to be the default mail client"},
 	{"mail.inappnotifications.enabled", "the vendor's in-app notices"},
 	{"mailnews.start_page.enabled", "the start page, loaded from the vendor at every start"},
+	{missedPref, "the reminders of meetings that have passed (novox/hq issue 640)"},
+}
+
+// missedPref is Betterbird's pass over missed reminders. With it on, the calendar's alarm service fires, at
+// every load of a calendar (each start, each refresh, each time Owl registers its calendar after signing in),
+// every reminder of the past month whose item carries no later acknowledgement (X-MOZ-LASTACK); with it off
+// (Betterbird's own default), a past reminder fires only while its meeting has not ended. Owl never sends a
+// dismissal to Exchange (its changes carry the reminder's offset alone) and keeps it only in Betterbird's local
+// cache, which a resynchronisation from scratch empties: a passed meeting's reminder then comes back at every
+// reload, Dismiss or not (novox/hq issue 640). The policy locks it off, so no setting and no profile can turn the
+// pass back on; betterbird_check says when the running Betterbird still has it on beside an Owl calendar.
+const missedPref = "calendar.alarms.showmissed"
+
+// missedReminders says why the running Betterbird shows a passed meeting's reminder again at each reload of an
+// Owl calendar, with the remedy, or two empty strings: Betterbird's pass over missed reminders on (the profile's
+// user.js, which wins at the start, else its prefs.js; Betterbird's default is off) while the profile holds an
+// Owl calendar and the placed policy did not lock it off before Betterbird started.
+func (m *Machine) missedReminders(profile string) (string, string) {
+	if profile == "" {
+		return "", ""
+	}
+	prefs := mergedPrefs(profile)
+	owl := 0
+	for _, c := range profileCalendars(prefs) {
+		if c.Kind == "owl" {
+			owl++
+		}
+	}
+	if owl == 0 {
+		return "", ""
+	}
+	value, from := "false", "Betterbird's default"
+	for _, f := range []string{"user.js", "prefs.js"} {
+		b, err := readBounded(filepath.Join(profile, f))
+		if err != nil {
+			continue
+		}
+		if v, ok := statementsOf(b)[missedPref]; ok {
+			value, from = v, "the profile's "+f
+			break
+		}
+	}
+	locked := m.locks()[missedPref] == "false"
+	ps := m.mainProcs()
+	if locked {
+		if len(ps) == 0 {
+			return "", ""
+		}
+		st, err := os.Stat(m.path(policyFile))
+		t, terr := time.Parse(time.RFC3339, ps[0].Since)
+		if err != nil || terr != nil || !st.ModTime().After(t) {
+			return "", ""
+		}
+	}
+	if value != "true" || len(ps) == 0 {
+		return "", ""
+	}
+	what := fmt.Sprintf("Betterbird shows the reminders of meetings that have passed again at every reload of its %d Owl "+
+		"calendar(s): %s is true (%s), and Owl keeps a dismissal only in Betterbird's local cache, never on the Exchange "+
+		"server (novox/hq issue 640)", owl, missedPref, from)
+	if locked {
+		return what, "betterbird_restart: the module's policy locks " + missedPref + " off, and the running Betterbird started before it was placed"
+	}
+	return what, "push the module to this node: its policy locks " + missedPref + " off; then betterbird_restart"
 }
 
 // policyProblem is everything the policy fails to say of what ADR 0316 and issue 345 ask, or "".
diff --git a/modules/betterbird/cmd/betterbird-tools/install_test.go b/modules/betterbird/cmd/betterbird-tools/install_test.go
index ffbb2884..2a80c2ab 100644
--- a/modules/betterbird/cmd/betterbird-tools/install_test.go
+++ b/modules/betterbird/cmd/betterbird-tools/install_test.go
@@ -229,3 +229,79 @@ func mustRead(t *testing.T, f *fake, p string) []byte {
 	}
 	return raw
 }
+
+// owlCalendar is an Owl calendar in the profile's prefs.js, as Owl registers its account's calendar.
+func (f *fake) owlCalendar(extra string) {
+	f.write(profileDir+"/prefs.js", "// Mozilla User Preferences\n"+
+		`user_pref("calendar.registry.45b5.type", "owl");`+"\n"+
+		`user_pref("calendar.registry.45b5.uri", "owl://someone%40example%2Eorg@outlook.example.org/");`+"\n"+
+		`user_pref("calendar.registry.45b5.name", "someone@example.org");`+"\n"+extra)
+}
+
+// A passed meeting's reminder shows at most once: the policy locks Betterbird's pass over missed reminders off,
+// whatever the setting prefs or the profile say, because Owl keeps a dismissal only in Betterbird's local cache
+// and the pass fires every unacknowledged reminder of the past month again at each reload (novox/hq issue 640).
+func TestAPassedMeetingsReminderIsNotShownAgainAtEachReloadOfAnOwlCalendar(t *testing.T) {
+	raw, err := os.ReadFile(filepath.Join("..", "..", "install", "distribution", "policies.json"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if p := policyPrefsOf(raw)[missedPref]; p.Value != "false" || !p.Locked {
+		t.Fatalf("the module's policy locks %s off: %+v", missedPref, p)
+	}
+	before := strings.Replace(string(raw), `"calendar.alarms.showmissed"`, `"calendar.alarms.unrelated"`, 1)
+	if why := policyProblem([]byte(before)); !strings.Contains(why, missedPref) {
+		t.Fatalf("a policy that leaves the missed reminders on is said: %q", why)
+	}
+
+	f := newFake(t)
+	f.betterbird()
+	f.placedByTheMesh()
+	links(t, nil)
+	nag := func() Finding {
+		t.Helper()
+		c, err := f.Check()
+		if err != nil {
+			t.Fatal(err)
+		}
+		for _, x := range c.Findings {
+			if strings.Contains(x.What, "Owl calendar") {
+				return x
+			}
+		}
+		return Finding{}
+	}
+	// g14 on 2026-10-13: the module's user.js turns the pass on, an Owl calendar is in the profile, and the
+	// placed policy does not lock it.
+	f.write(policyFile, before)
+	f.owlCalendar("")
+	f.write(profileDir+"/user.js", mark+")\n"+`user_pref("calendar.alarms.showmissed", true);`+"\n")
+	if x := nag(); !strings.Contains(x.What, "1 Owl calendar") || !strings.Contains(x.What, "user.js") ||
+		!strings.Contains(x.Do, "push the module") {
+		t.Fatalf("the nag is a finding with its remedy: %+v", x)
+	}
+	// The policy placed after the running Betterbird started: in effect from its next start.
+	f.write(policyFile, string(raw))
+	later := time.Unix(1_900_000_000, 0)
+	os.Chtimes(filepath.Join(f.Root, policyFile), later, later)
+	if x := nag(); !strings.Contains(x.Do, "betterbird_restart") || strings.Contains(x.Do, "push") {
+		t.Fatalf("a lock placed after the start waits for a restart: %+v", x)
+	}
+	// The lock in effect: nothing the profile says turns the pass back on.
+	f.write(policyFile, string(raw))
+	f.owlCalendar(`user_pref("calendar.alarms.showmissed", true);` + "\n")
+	if x := nag(); x.What != "" {
+		t.Fatalf("the lock holds whatever the profile says: %+v", x)
+	}
+	// Without the lock, the pass off is no nag, and neither is the pass on without an Owl calendar.
+	f.write(policyFile, before)
+	f.write(profileDir+"/user.js", mark+")\n"+`user_pref("calendar.alarms.showmissed", false);`+"\n")
+	if x := nag(); x.What != "" {
+		t.Fatalf("the pass off: %+v", x)
+	}
+	f.write(profileDir+"/user.js", mark+")\n"+`user_pref("calendar.alarms.showmissed", true);`+"\n")
+	f.write(profileDir+"/prefs.js", `user_pref("calendar.registry.c1.type", "caldav");`+"\n"+`user_pref("calendar.registry.c1.uri", "https://dav.example.org/c1/");`+"\n")
+	if x := nag(); x.What != "" {
+		t.Fatalf("a CalDAV calendar keeps its dismissals on its server: %+v", x)
+	}
+}
diff --git a/modules/betterbird/install/distribution/policies.json b/modules/betterbird/install/distribution/policies.json
index 4edb6abe..600b9c7a 100644
--- a/modules/betterbird/install/distribution/policies.json
+++ b/modules/betterbird/install/distribution/policies.json
@@ -22,6 +22,10 @@
       "mailnews.start_page.enabled": {
         "Value": false,
         "Status": "locked"
+      },
+      "calendar.alarms.showmissed": {
+        "Value": false,
+        "Status": "locked"
       }
     }
   }