diff --git a/src/ZB.MOM.WW.ScadaBridge.NotificationOutbox/Kpi/NotificationOutboxKpiSampleSource.cs b/src/ZB.MOM.WW.ScadaBridge.NotificationOutbox/Kpi/NotificationOutboxKpiSampleSource.cs
new file mode 100644
index 00000000..e678a116
--- /dev/null
+++ b/src/ZB.MOM.WW.ScadaBridge.NotificationOutbox/Kpi/NotificationOutboxKpiSampleSource.cs
@@ -0,0 +1,142 @@
+using Microsoft.Extensions.Options;
+using ZB.MOM.WW.ScadaBridge.Commons.Entities.Kpi;
+using ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Kpi;
+using ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Repositories;
+using ZB.MOM.WW.ScadaBridge.Commons.Types.Kpi;
+using ZB.MOM.WW.ScadaBridge.Commons.Types.Notifications;
+
+namespace ZB.MOM.WW.ScadaBridge.NotificationOutbox.Kpi;
+
+/// <summary>
+/// <see cref="IKpiSampleSource"/> for the Notification Outbox (#21): snapshots the same
+/// point-in-time delivery KPIs the live Health-dashboard tiles surface — queue depth, stuck
+/// count, parked count, delivered-last-interval, and oldest-pending age — at every sampling
+/// pass of the central KPI-history recorder (M6 "KPI History &amp; Trends").
+/// </summary>
+/// <remarks>
+/// <para>
+/// Computes the same cutoffs the live KPI handlers in
+/// <see cref="NotificationOutboxActor"/> use, anchored on the recorder's shared
+/// <c>capturedAtUtc</c> rather than wall-clock <c>now</c>: the stuck cutoff is
+/// <c>capturedAtUtc - <see cref="NotificationOutboxOptions.StuckAgeThreshold"/></c> and the
+/// delivered window is <c>capturedAtUtc - <see cref="NotificationOutboxOptions.DeliveredKpiWindow"/></c>.
+/// So a sample captured at the same instant equals the live tile.
+/// </para>
+/// <para>
+/// Emits Global (<c>ScopeKey == null</c>), per-Site (<c>ScopeKey == SourceSiteId</c>), and
+/// per-Node (<c>ScopeKey == SourceNode</c>) samples, mirroring the repository's three KPI
+/// computation methods. The oldest-pending age (a <see cref="TimeSpan"/>?) maps to
+/// <c>oldestPendingAgeSeconds</c> via <see cref="TimeSpan.TotalSeconds"/>; that one metric is
+/// omitted when the age is <c>null</c>.
+/// </para>
+/// </remarks>
+public sealed class NotificationOutboxKpiSampleSource : IKpiSampleSource
+{
+    private const string MetricQueueDepth = "queueDepth";
+    private const string MetricStuckCount = "stuckCount";
+    private const string MetricParkedCount = "parkedCount";
+    private const string MetricDeliveredLastInterval = "deliveredLastInterval";
+    private const string MetricOldestPendingAgeSeconds = "oldestPendingAgeSeconds";
+
+    private readonly INotificationOutboxRepository _repository;
+    private readonly NotificationOutboxOptions _options;
+
+    /// <summary>
+    /// Creates the sample source.
+    /// </summary>
+    /// <param name="repository">Outbox repository providing the KPI computation methods.</param>
+    /// <param name="options">Outbox options carrying the stuck-age and delivered-window cutoffs.</param>
+    public NotificationOutboxKpiSampleSource(
+        INotificationOutboxRepository repository,
+        IOptions<NotificationOutboxOptions> options)
+    {
+        ArgumentNullException.ThrowIfNull(repository);
+        ArgumentNullException.ThrowIfNull(options);
+
+        _repository = repository;
+        _options = options.Value;
+    }
+
+    /// <inheritdoc />
+    public string Source => KpiSources.NotificationOutbox;
+
+    /// <inheritdoc />
+    public async Task<IReadOnlyList<KpiSample>> CollectAsync(
+        DateTime capturedAtUtc, CancellationToken cancellationToken = default)
+    {
+        // Anchor the live KPI cutoffs on the recorder's shared capture instant, so a sample
+        // captured at the same moment equals the live Health-dashboard tile.
+        var capturedAt = new DateTimeOffset(capturedAtUtc, TimeSpan.Zero);
+        var stuckCutoff = capturedAt - _options.StuckAgeThreshold;
+        var deliveredSince = capturedAt - _options.DeliveredKpiWindow;
+
+        var samples = new List<KpiSample>();
+
+        var global = await _repository.ComputeKpisAsync(stuckCutoff, deliveredSince, cancellationToken)
+            .ConfigureAwait(false);
+        AddSnapshot(
+            samples, capturedAtUtc, KpiScopes.Global, scopeKey: null,
+            global.QueueDepth, global.StuckCount, global.ParkedCount,
+            global.DeliveredLastInterval, global.OldestPendingAge);
+
+        var perSite = await _repository.ComputePerSiteKpisAsync(stuckCutoff, deliveredSince, cancellationToken)
+            .ConfigureAwait(false);
+        foreach (var site in perSite)
+        {
+            AddSnapshot(
+                samples, capturedAtUtc, KpiScopes.Site, site.SourceSiteId,
+                site.QueueDepth, site.StuckCount, site.ParkedCount,
+                site.DeliveredLastInterval, site.OldestPendingAge);
+        }
+
+        var perNode = await _repository.ComputePerNodeKpisAsync(stuckCutoff, deliveredSince, cancellationToken)
+            .ConfigureAwait(false);
+        foreach (var node in perNode)
+        {
+            AddSnapshot(
+                samples, capturedAtUtc, KpiScopes.Node, node.SourceNode,
+                node.QueueDepth, node.StuckCount, node.ParkedCount,
+                node.DeliveredLastInterval, node.OldestPendingAge);
+        }
+
+        return samples;
+    }
+
+    /// <summary>
+    /// Appends the five outbox metrics for one snapshot at the given scope, omitting
+    /// <c>oldestPendingAgeSeconds</c> when <paramref name="oldestPendingAge"/> is <c>null</c>.
+    /// </summary>
+    private void AddSnapshot(
+        List<KpiSample> samples,
+        DateTime capturedAtUtc,
+        string scope,
+        string? scopeKey,
+        int queueDepth,
+        int stuckCount,
+        int parkedCount,
+        int deliveredLastInterval,
+        TimeSpan? oldestPendingAge)
+    {
+        samples.Add(Sample(capturedAtUtc, MetricQueueDepth, scope, scopeKey, queueDepth));
+        samples.Add(Sample(capturedAtUtc, MetricStuckCount, scope, scopeKey, stuckCount));
+        samples.Add(Sample(capturedAtUtc, MetricParkedCount, scope, scopeKey, parkedCount));
+        samples.Add(Sample(capturedAtUtc, MetricDeliveredLastInterval, scope, scopeKey, deliveredLastInterval));
+
+        if (oldestPendingAge is { } age)
+        {
+            samples.Add(Sample(capturedAtUtc, MetricOldestPendingAgeSeconds, scope, scopeKey, age.TotalSeconds));
+        }
+    }
+
+    private KpiSample Sample(
+        DateTime capturedAtUtc, string metric, string scope, string? scopeKey, double value) =>
+        new()
+        {
+            Source = KpiSources.NotificationOutbox,
+            Metric = metric,
+            Scope = scope,
+            ScopeKey = scopeKey,
+            Value = value,
+            CapturedAtUtc = capturedAtUtc,
+        };
+}
diff --git a/src/ZB.MOM.WW.ScadaBridge.NotificationOutbox/ServiceCollectionExtensions.cs b/src/ZB.MOM.WW.ScadaBridge.NotificationOutbox/ServiceCollectionExtensions.cs
index 009a42dc..8d8912b8 100644
--- a/src/ZB.MOM.WW.ScadaBridge.NotificationOutbox/ServiceCollectionExtensions.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.NotificationOutbox/ServiceCollectionExtensions.cs
@@ -1,5 +1,7 @@
 using Microsoft.Extensions.DependencyInjection;
+using ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Kpi;
 using ZB.MOM.WW.ScadaBridge.NotificationOutbox.Delivery;
+using ZB.MOM.WW.ScadaBridge.NotificationOutbox.Kpi;
 
 namespace ZB.MOM.WW.ScadaBridge.NotificationOutbox;
 
@@ -46,6 +48,10 @@ public static class ServiceCollectionExtensions
         services.AddScoped<INotificationDeliveryAdapter>(
             sp => sp.GetRequiredService<EmailNotificationDeliveryAdapter>());
 
+        // KPI history (M6): the recorder singleton enumerates every IKpiSampleSource each
+        // sampling pass to snapshot the outbox delivery KPIs into the central history store.
+        services.AddScoped<IKpiSampleSource, NotificationOutboxKpiSampleSource>();
+
         return services;
     }
 }
diff --git a/tests/ZB.MOM.WW.ScadaBridge.NotificationService.Tests/Kpi/NotificationOutboxKpiSampleSourceTests.cs b/tests/ZB.MOM.WW.ScadaBridge.NotificationService.Tests/Kpi/NotificationOutboxKpiSampleSourceTests.cs
new file mode 100644
index 00000000..c7f24d50
--- /dev/null
+++ b/tests/ZB.MOM.WW.ScadaBridge.NotificationService.Tests/Kpi/NotificationOutboxKpiSampleSourceTests.cs
@@ -0,0 +1,182 @@
+using Microsoft.Extensions.Options;
+using NSubstitute;
+using ZB.MOM.WW.ScadaBridge.Commons.Entities.Kpi;
+using ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Repositories;
+using ZB.MOM.WW.ScadaBridge.Commons.Types.Kpi;
+using ZB.MOM.WW.ScadaBridge.Commons.Types.Notifications;
+using ZB.MOM.WW.ScadaBridge.NotificationOutbox;
+using ZB.MOM.WW.ScadaBridge.NotificationOutbox.Kpi;
+
+namespace ZB.MOM.WW.ScadaBridge.NotificationService.Tests.Kpi;
+
+/// <summary>
+/// Tests for <see cref="NotificationOutboxKpiSampleSource"/> — the M6 KPI sample source that
+/// snapshots the Notification Outbox delivery KPIs (global / per-site / per-node) into the
+/// central KPI-history store.
+/// </summary>
+public class NotificationOutboxKpiSampleSourceTests
+{
+    private static readonly DateTime CapturedAt = new(2026, 6, 15, 12, 0, 0, DateTimeKind.Utc);
+
+    private static readonly NotificationOutboxOptions Options = new()
+    {
+        StuckAgeThreshold = TimeSpan.FromMinutes(10),
+        DeliveredKpiWindow = TimeSpan.FromMinutes(1),
+    };
+
+    private static NotificationOutboxKpiSampleSource CreateSource(INotificationOutboxRepository repository) =>
+        new(repository, Microsoft.Extensions.Options.Options.Create(Options));
+
+    [Fact]
+    public void Source_IsNotificationOutbox()
+    {
+        var source = CreateSource(Substitute.For<INotificationOutboxRepository>());
+
+        Assert.Equal(KpiSources.NotificationOutbox, source.Source);
+    }
+
+    [Fact]
+    public async Task CollectAsync_PassesCutoffsAnchoredOnCapturedAt()
+    {
+        var repository = Substitute.For<INotificationOutboxRepository>();
+        StubEmptySnapshots(repository);
+
+        var source = CreateSource(repository);
+        await source.CollectAsync(CapturedAt);
+
+        var expectedStuckCutoff = new DateTimeOffset(CapturedAt, TimeSpan.Zero) - Options.StuckAgeThreshold;
+        var expectedDeliveredSince = new DateTimeOffset(CapturedAt, TimeSpan.Zero) - Options.DeliveredKpiWindow;
+
+        await repository.Received(1).ComputeKpisAsync(
+            expectedStuckCutoff, expectedDeliveredSince, Arg.Any<CancellationToken>());
+        await repository.Received(1).ComputePerSiteKpisAsync(
+            expectedStuckCutoff, expectedDeliveredSince, Arg.Any<CancellationToken>());
+        await repository.Received(1).ComputePerNodeKpisAsync(
+            expectedStuckCutoff, expectedDeliveredSince, Arg.Any<CancellationToken>());
+    }
+
+    [Fact]
+    public async Task CollectAsync_EmitsGlobalSiteAndNodeSamples_WithExpectedTuples()
+    {
+        var repository = Substitute.For<INotificationOutboxRepository>();
+        repository.ComputeKpisAsync(Arg.Any<DateTimeOffset>(), Arg.Any<DateTimeOffset>(), Arg.Any<CancellationToken>())
+            .Returns(new NotificationKpiSnapshot(
+                QueueDepth: 5,
+                StuckCount: 2,
+                ParkedCount: 1,
+                DeliveredLastInterval: 7,
+                OldestPendingAge: TimeSpan.FromSeconds(90)));
+        repository.ComputePerSiteKpisAsync(Arg.Any<DateTimeOffset>(), Arg.Any<DateTimeOffset>(), Arg.Any<CancellationToken>())
+            .Returns(new[]
+            {
+                new SiteNotificationKpiSnapshot(
+                    SourceSiteId: "site-a",
+                    QueueDepth: 3,
+                    StuckCount: 1,
+                    ParkedCount: 0,
+                    DeliveredLastInterval: 4,
+                    OldestPendingAge: TimeSpan.FromSeconds(30)),
+            });
+        repository.ComputePerNodeKpisAsync(Arg.Any<DateTimeOffset>(), Arg.Any<DateTimeOffset>(), Arg.Any<CancellationToken>())
+            .Returns(new[]
+            {
+                new NodeNotificationKpiSnapshot(
+                    SourceNode: "node-a",
+                    QueueDepth: 2,
+                    StuckCount: 1,
+                    ParkedCount: 1,
+                    DeliveredLastInterval: 3,
+                    OldestPendingAge: TimeSpan.FromSeconds(60)),
+            });
+
+        var source = CreateSource(repository);
+        var samples = await source.CollectAsync(CapturedAt);
+
+        // 3 scopes × 5 metrics (all ages non-null) = 15 samples.
+        Assert.Equal(15, samples.Count);
+        Assert.All(samples, s => Assert.Equal(KpiSources.NotificationOutbox, s.Source));
+        Assert.All(samples, s => Assert.Equal(CapturedAt, s.CapturedAtUtc));
+
+        // Global — null ScopeKey.
+        AssertSample(samples, "queueDepth", KpiScopes.Global, null, 5);
+        AssertSample(samples, "stuckCount", KpiScopes.Global, null, 2);
+        AssertSample(samples, "parkedCount", KpiScopes.Global, null, 1);
+        AssertSample(samples, "deliveredLastInterval", KpiScopes.Global, null, 7);
+        AssertSample(samples, "oldestPendingAgeSeconds", KpiScopes.Global, null, 90);
+
+        // Site — ScopeKey == site id.
+        AssertSample(samples, "queueDepth", KpiScopes.Site, "site-a", 3);
+        AssertSample(samples, "stuckCount", KpiScopes.Site, "site-a", 1);
+        AssertSample(samples, "parkedCount", KpiScopes.Site, "site-a", 0);
+        AssertSample(samples, "deliveredLastInterval", KpiScopes.Site, "site-a", 4);
+        AssertSample(samples, "oldestPendingAgeSeconds", KpiScopes.Site, "site-a", 30);
+
+        // Node — ScopeKey == node name.
+        AssertSample(samples, "queueDepth", KpiScopes.Node, "node-a", 2);
+        AssertSample(samples, "stuckCount", KpiScopes.Node, "node-a", 1);
+        AssertSample(samples, "parkedCount", KpiScopes.Node, "node-a", 1);
+        AssertSample(samples, "deliveredLastInterval", KpiScopes.Node, "node-a", 3);
+        AssertSample(samples, "oldestPendingAgeSeconds", KpiScopes.Node, "node-a", 60);
+    }
+
+    [Fact]
+    public async Task CollectAsync_OmitsOldestPendingAge_WhenNull()
+    {
+        var repository = Substitute.For<INotificationOutboxRepository>();
+        repository.ComputeKpisAsync(Arg.Any<DateTimeOffset>(), Arg.Any<DateTimeOffset>(), Arg.Any<CancellationToken>())
+            .Returns(new NotificationKpiSnapshot(
+                QueueDepth: 0,
+                StuckCount: 0,
+                ParkedCount: 0,
+                DeliveredLastInterval: 0,
+                OldestPendingAge: null));
+        repository.ComputePerSiteKpisAsync(Arg.Any<DateTimeOffset>(), Arg.Any<DateTimeOffset>(), Arg.Any<CancellationToken>())
+            .Returns(Array.Empty<SiteNotificationKpiSnapshot>());
+        repository.ComputePerNodeKpisAsync(Arg.Any<DateTimeOffset>(), Arg.Any<DateTimeOffset>(), Arg.Any<CancellationToken>())
+            .Returns(Array.Empty<NodeNotificationKpiSnapshot>());
+
+        var source = CreateSource(repository);
+        var samples = await source.CollectAsync(CapturedAt);
+
+        // Only the global snapshot, age omitted -> the four count metrics.
+        Assert.Equal(4, samples.Count);
+        Assert.DoesNotContain(samples, s => s.Metric == "oldestPendingAgeSeconds");
+        AssertSample(samples, "queueDepth", KpiScopes.Global, null, 0);
+        AssertSample(samples, "stuckCount", KpiScopes.Global, null, 0);
+        AssertSample(samples, "parkedCount", KpiScopes.Global, null, 0);
+        AssertSample(samples, "deliveredLastInterval", KpiScopes.Global, null, 0);
+    }
+
+    [Fact]
+    public async Task CollectAsync_ReturnsEmptyList_NeverNull_WhenNothingToReport()
+    {
+        // ComputeKpisAsync always returns a global snapshot; the only way the list is empty is
+        // a guard that produces no samples. Confirm an all-zero global with null age still yields
+        // the four count metrics (i.e. the list is never null even at idle).
+        var repository = Substitute.For<INotificationOutboxRepository>();
+        StubEmptySnapshots(repository);
+
+        var source = CreateSource(repository);
+        var samples = await source.CollectAsync(CapturedAt);
+
+        Assert.NotNull(samples);
+    }
+
+    private static void StubEmptySnapshots(INotificationOutboxRepository repository)
+    {
+        repository.ComputeKpisAsync(Arg.Any<DateTimeOffset>(), Arg.Any<DateTimeOffset>(), Arg.Any<CancellationToken>())
+            .Returns(new NotificationKpiSnapshot(0, 0, 0, 0, null));
+        repository.ComputePerSiteKpisAsync(Arg.Any<DateTimeOffset>(), Arg.Any<DateTimeOffset>(), Arg.Any<CancellationToken>())
+            .Returns(Array.Empty<SiteNotificationKpiSnapshot>());
+        repository.ComputePerNodeKpisAsync(Arg.Any<DateTimeOffset>(), Arg.Any<DateTimeOffset>(), Arg.Any<CancellationToken>())
+            .Returns(Array.Empty<NodeNotificationKpiSnapshot>());
+    }
+
+    private static void AssertSample(
+        IReadOnlyList<KpiSample> samples, string metric, string scope, string? scopeKey, double value)
+    {
+        var match = Assert.Single(
+            samples, s => s.Metric == metric && s.Scope == scope && s.ScopeKey == scopeKey);
+        Assert.Equal(value, match.Value);
+    }
+}
diff --git a/tests/ZB.MOM.WW.ScadaBridge.NotificationService.Tests/ZB.MOM.WW.ScadaBridge.NotificationService.Tests.csproj b/tests/ZB.MOM.WW.ScadaBridge.NotificationService.Tests/ZB.MOM.WW.ScadaBridge.NotificationService.Tests.csproj
index 3a18ae53..ec9170b7 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.NotificationService.Tests/ZB.MOM.WW.ScadaBridge.NotificationService.Tests.csproj
+++ b/tests/ZB.MOM.WW.ScadaBridge.NotificationService.Tests/ZB.MOM.WW.ScadaBridge.NotificationService.Tests.csproj
@@ -23,6 +23,7 @@
 
   <ItemGroup>
     <ProjectReference Include="../../src/ZB.MOM.WW.ScadaBridge.NotificationService/ZB.MOM.WW.ScadaBridge.NotificationService.csproj" />
+    <ProjectReference Include="../../src/ZB.MOM.WW.ScadaBridge.NotificationOutbox/ZB.MOM.WW.ScadaBridge.NotificationOutbox.csproj" />
     <ProjectReference Include="../../src/ZB.MOM.WW.ScadaBridge.Commons/ZB.MOM.WW.ScadaBridge.Commons.csproj" />
     <ProjectReference Include="../../src/ZB.MOM.WW.ScadaBridge.StoreAndForward/ZB.MOM.WW.ScadaBridge.StoreAndForward.csproj" />
   </ItemGroup>