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Why suppression matters

When you send a tracked email and then open it yourself (e.g., in your Sent folder), the tracking pixel fires just like it would for a recipient. Without suppression, your own opens would inflate the open count. Email Tracker uses identity-based, event-driven suppression to reliably distinguish sender opens from genuine recipient opens.

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How it works

The suppression system uses a two-phase approach:
  1. Detection phase - Content script scans visible emails for tracking pixels and identifies sender-owned messages
  2. Suppression phase - Server receives suppression signal and marks the next pixel hit as sender-suppressed
Sender suppression flow diagram

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Detection phase

The Gmail content script continuously scans for tracking pixels in visible messages.

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Scanning for tracking pixels

// extension/src/content/gmailCompose.js:519-570
function scanAndMarkSuppressNext() {
  if (!isRuntimeAvailable()) {
    return;
  }

  if (!isConversationViewRendered()) {
    return;
  }

  const currentEmail = getCurrentLoggedInEmail();
  if (!isLikelyEmail(currentEmail)) {
    return;
  }

  const images = document.querySelectorAll("img[src]");
  images.forEach((imgNode) => {
    const src = imgNode.getAttribute("src") || imgNode.src || "";
    const token = extractTokenFromTrackingSrc(src);
    if (!token) {
      return;
    }

    const payload = decodeTrackingPayloadFromToken(token);
    if (!payload?.emailId || !payload?.senderEmail) {
      return;
    }

    if (processedSuppressEmailIds.has(payload.emailId)) {
      return;
    }

    // Identity-based suppression: sender viewing own tracked message
    if (normalizeEmailCandidate(payload.senderEmail) !== normalizeEmailCandidate(currentEmail)) {
      return;
    }

    processedSuppressEmailIds.add(payload.emailId);
    chrome.runtime.sendMessage({
      type: "tracker:markSuppressNext",
      emailId: payload.emailId
    });
  });
}

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Identity comparison

The key to reliable suppression is comparing the sender email in the token with the currently logged-in Gmail account: 
// extension/src/content/gmailCompose.js:554-558
// Identity-based suppression: sender viewing own tracked message should suppress next open.
// Folder names are unreliable in Gmail SPA; account identity is stable for this decision.
if (normalizeEmailCandidate(payload.senderEmail) !== normalizeEmailCandidate(currentEmail)) {
  return;
}
This approach is more reliable than folder-based detection (e.g., checking if you’re in “Sent” folder) because Gmail’s SPA architecture makes folder inference brittle. Account identity is stable and unambiguous.

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Token decoding in extension

The content script decodes tracking tokens client-side to extract sender email: 
// extension/src/content/gmailCompose.js:674-705
function decodeTrackingPayloadFromToken(token) {
  const raw = String(token || "").trim();
  if (!raw) {
    return null;
  }

  try {
    const normalized = raw.replace(/-/g, "+").replace(/_/g, "/");
    const padded = normalized + "=".repeat((4 - (normalized.length % 4)) % 4);
    const binary = atob(padded);
    const bytes = Uint8Array.from(binary, (char) => char.charCodeAt(0));
    const json = new TextDecoder().decode(bytes);
    const parsed = JSON.parse(json);

    if (Array.isArray(parsed)) {
      const emailId = String(parsed[1] || "").trim().toLowerCase();
      const senderEmail = normalizeEmailCandidate(parsed[4] || "");
      return emailId ? { emailId, senderEmail } : null;
    }

    if (parsed && typeof parsed === "object") {
      const emailId = String(parsed.email_id || "").trim().toLowerCase();
      const senderEmail = normalizeEmailCandidate(parsed.sender_email || "");
      return emailId ? { emailId, senderEmail } : null;
    }

    return null;
  } catch {
    return null;
  }
}

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Suppression phase

Once the content script detects a sender-owned message, it signals the server to suppress the next pixel hit for that email_id.

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Suppression signal endpoint

The background worker sends a POST /mark-suppress-next request: 
// extension/src/background/serviceWorker.js:324-350
async function markSuppressNextForEmail(emailId) {
  if (!emailId) {
    return { sent: false, reason: "missing email id" };
  }

  const normalizedBaseUrl = await getTrackerBaseUrl();

  try {
    const response = await fetch(`${normalizedBaseUrl}/mark-suppress-next`, {
      method: "POST",
      headers: {
        "Content-Type": "application/json"
      },
      body: JSON.stringify({ email_id: emailId }),
      cache: "no-store",
      credentials: "omit"
    });

    if (!response.ok) {
      return { sent: false, reason: `http ${response.status}` };
    }

    return { sent: true, reason: "ok" };
  } catch (error) {
    return { sent: false, reason: String(error?.message || error) };
  }
}

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Server-side suppression map

The server maintains an in-memory map of pending suppression signals: 
// server/src/routes/track.ts:13-32
interface SuppressionEntry {
  createdAtMs: number;
}

interface SuppressionDebugEvent {
  event: "mark_suppress_next" | "google_proxy_hit" | "suppression_consumed" | "suppression_expired";
  email_id: string;
  at_ms: number;
  ip: string;
  user_agent: string;
  delta_ms?: number;
  pending_suppression?: boolean;
}

// Event-based, email-scoped suppression store with consume-once semantics.
// TTL exists only as stale-entry cleanup fallback, not suppression logic.
const suppressionMap = new Map<string, SuppressionEntry>();
const latencySamples: number[] = [];
const suppressionDebugEvents: SuppressionDebugEvent[] = [];
let suppressSignalCount = 0;

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Marking an email for suppression

When the server receives a suppression signal, it stores a timestamp: 
// server/src/routes/track.ts:36-91
trackRouter.post("/mark-suppress-next", (req, res) => {
  const nowMs = Date.now();
  cleanupExpiredSuppressions(nowMs);

  const signal = markSuppressNext(req.body?.email_id, req, nowMs);
  if (!signal.ok) {
    res.status(400).json({ ok: false, error: "email_id is required" });
    return;
  }

  res.json({ ok: true, email_id: signal.emailId, recorded_at_ms: signal.recordedAtMs });
});

function markSuppressNext(
  rawEmailId: unknown,
  req: { /* ... */ },
  nowMs: number
): { ok: true; emailId: string; recordedAtMs: number } | { ok: false } {
  const emailId = String(rawEmailId || "").trim();
  if (!emailId) {
    return { ok: false };
  }

  suppressionMap.set(emailId, { createdAtMs: nowMs });
  enforceSuppressionMapLimit();

  suppressSignalCount += 1;
  pushSuppressionDebugEvent({
    event: "mark_suppress_next",
    email_id: emailId,
    at_ms: nowMs,
    ip: normalizeIp(getRequestIp(req)),
    user_agent: String(req.get?.("user-agent") || "")
  });

  console.info(
    JSON.stringify({
      event: "suppress_signal_received",
      endpoint: req.path,
      email_id: emailId,
      at_ms: nowMs,
      map_size: suppressionMap.size
    })
  );

  return { ok: true, emailId, recordedAtMs: nowMs };
}

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Consuming suppression signal

When the pixel endpoint receives a request, it checks for pending suppression: 
// server/src/routes/track.ts:93-160
trackRouter.get("/t/:token.gif", (req, res) => {
  const nowMs = Date.now();
  const openedAtIso = new Date(nowMs).toISOString();
  const token = req.params.token;

  try {
    const payload = decodeTrackingToken(token);
    const ipAddress = getRequestIp(req);
    const userAgent = req.get("user-agent") || null;
    const emailId = payload.email_id;

    cleanupExpiredSuppressions(nowMs);

    const pendingSuppression = suppressionMap.get(emailId);
    const wasSuppressedBySignal = Boolean(pendingSuppression);
    const deltaMs = pendingSuppression ? Math.max(0, nowMs - pendingSuppression.createdAtMs) : null;

    if (pendingSuppression) {
      suppressionMap.delete(emailId);  // Consume once
      pushSuppressionDebugEvent({
        event: "suppression_consumed",
        email_id: emailId,
        at_ms: nowMs,
        ip: normalizeIp(ipAddress),
        user_agent: String(userAgent || ""),
        delta_ms: deltaMs ?? undefined
      });
    }

    const result = recordOpenEvent({
      payload,
      ipAddress,
      userAgent,
      openedAtIso,
      forceSenderSuppressed: wasSuppressedBySignal,
      suppressionReason: wasSuppressedBySignal ? "mark_suppress_next" : null
    });

    // ...
  } catch (error) {
    console.error("Tracking pixel processing failed:", error);
  }

  // Always return 200 + GIF
  res.status(200).send(TRANSPARENT_PIXEL_GIF);
});
Suppression signals are consumed once (line 111: suppressionMap.delete(emailId)). If the pixel fires again later (e.g., sender reopens the message), subsequent opens are not suppressed.

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Consume-once semantics

The suppression system uses consume-once semantics:
  1. Content script sends suppression signal: POST /mark-suppress-next
  2. Server stores { email_id: timestamp } in memory
  3. When pixel fires, server checks for pending suppression
  4. If found, server deletes the entry and marks event as suppressed
  5. Subsequent pixel hits for that email_id are not suppressed
1

Signal sent

Sender opens message → Content script sends POST /mark-suppress-next → Server stores suppressionMap.set(email_id, { createdAtMs })
2

Pixel fires

Email client requests pixel → Server finds email_id in suppressionMap → Server deletes entry and marks event as suppressed
3

Subsequent opens

Sender or recipient opens again → No suppression entry exists → Open is counted normally

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Suppression TTL

Suppression entries have a 10-second TTL as a cleanup fallback, not as primary suppression logic: 
// server/src/routes/track.ts:6
const SUPPRESSION_TTL_MS = 10_000;

// server/src/routes/track.ts:283-298
function cleanupExpiredSuppressions(nowMs: number): void {
  for (const [emailId, entry] of suppressionMap.entries()) {
    if (nowMs - entry.createdAtMs <= SUPPRESSION_TTL_MS) {
      continue;
    }

    suppressionMap.delete(emailId);
    pushSuppressionDebugEvent({
      event: "suppression_expired",
      email_id: emailId,
      at_ms: nowMs,
      ip: "",
      user_agent: ""
    });
  }
}
The 10-second TTL exists to prevent memory leaks if pixel requests never arrive. It’s not a “suppression window” — suppression is consumed immediately when the pixel fires.

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Why this is reliable for Gmail

The identity-based approach has key advantages over other suppression methods:
Gmail’s SPA architecture makes it unreliable to detect whether you’re viewing “Sent” vs “Inbox”. URL paths and DOM structure change frequently.Identity comparison is stable: The logged-in account email is always available and unambiguous.
Whether you open your message from:
  • Sent folder
  • Search results
  • Conversation view
  • Pop-out window
The identity check (sender_email === logged_in_email) works consistently.
Some trackers use a “suppression window” where all opens within N seconds are suppressed. This can miss legitimate recipient opens.Email Tracker uses explicit signal + consume-once, so only the first pixel hit after the signal is suppressed.
If you send from a shared inbox (e.g., [email protected]) and a teammate opens the message while logged in as [email protected], the open is not suppressed because sender_email !== [email protected].

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Gmail Image Proxy latency

Gmail proxies images through Google’s servers, introducing latency between the suppression signal and pixel hit. The tracker measures this latency for debugging.

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Latency sampling

// server/src/routes/track.ts:122-151
const isGoogleProxyHit = isGoogleImageProxyHit(userAgent, ipAddress);
if (isGoogleProxyHit) {
  pushSuppressionDebugEvent({
    event: "google_proxy_hit",
    email_id: emailId,
    at_ms: nowMs,
    ip: normalizeIp(ipAddress),
    user_agent: String(userAgent || ""),
    pending_suppression: wasSuppressedBySignal,
    delta_ms: deltaMs ?? undefined
  });

  if (wasSuppressedBySignal && typeof deltaMs === "number") {
    latencySamples.push(deltaMs);
    if (latencySamples.length > LATENCY_SAMPLE_LIMIT) {
      latencySamples.splice(0, latencySamples.length - LATENCY_SAMPLE_LIMIT);
    }

    console.info(
      JSON.stringify({
        event: "gmail_proxy_latency_sample",
        email_id: emailId,
        delta_ms: deltaMs,
        user_agent: userAgent || "",
        ip: normalizeIp(ipAddress)
      })
    );
  }
}

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Latency metrics endpoint

View Gmail proxy latency statistics: 
curl http://localhost:8090/metrics/gmail-proxy-latency
Response: 
{
  "count": 42,
  "min": 245,
  "max": 3821,
  "avg": 1203.45,
  "p50": 1150,
  "p90": 2100,
  "p95": 2450,
  "p99": 3200
}

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Debug endpoints

The tracker provides debug endpoints to troubleshoot suppression:

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Suppression signals

curl http://localhost:8090/metrics/suppress-signals
Returns: 
{
  "count": 156,
  "active_email_ids": 3,
  "ttl_ms": 10000,
  "recent": [
    {
      "event": "mark_suppress_next",
      "email_id": "abc-123",
      "at_ms": 1709845123456,
      "ip": "192.168.1.1",
      "user_agent": "Mozilla/5.0..."
    }
  ]
}

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Suppression debug

curl http://localhost:8090/metrics/suppression-debug
Returns per-email breakdown: 
{
  "active_email_ids": 3,
  "ttl_ms": 10000,
  "recent_events": [...],
  "by_email": {
    "abc-123": {
      "marks": [1709845123456],
      "google_proxy_hits": [1709845124789],
      "consumed": [1709845124789],
      "expired": []
    }
  }
}

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Database flags

Suppressed opens are stored in the database with flags: 
-- server/src/db/schema.sql:28-29
is_sender_suppressed INTEGER NOT NULL DEFAULT 0 CHECK (is_sender_suppressed IN (0, 1)),
suppression_reason TEXT,
Suppressed events are stored for audit/debug but do not increment tracked_emails.open_count: 
// server/src/services/openRecorder.ts:161-163
if (!isDuplicate && !isSenderSuppressed) {
  incrementOpenCountStmt.run(input.payload.email_id);
}

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Frequently asked questions

Only the first pixel hit after the suppression signal is suppressed. Subsequent opens are counted normally (but may be caught by deduplication).
If the pixel request arrives before the POST /mark-suppress-next signal, the open is counted. This can happen if:
  • Network latency is high
  • Gmail’s image proxy is very fast
  • The extension content script is slow to scan
In practice, the signal usually arrives first because it’s sent immediately when the content script detects the pixel.
No. Suppression is sender-based (not recipient-based). It only suppresses opens when the sender views their own message.To implement recipient-based suppression, you’d need to maintain a blocklist of recipient emails/IPs.
No. If sender_email is not included in the tracking token, the content script cannot compare it with the logged-in account, so suppression does not trigger.Ensure your extension captures sender email during token generation:
// extension/src/background/serviceWorker.js:26
const senderEmail = String(message.senderEmail || "").trim().toLowerCase() || null;

Email tracking

Learn how pixel tracking works end-to-end

Deduplication

Understand how duplicate opens are detected

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