Aug 24, 2026 · 9 min read
Is Sailthru Tracking Your Email? How to Block the Beacon
Sailthru sends a large share of publisher and retail newsletters. Its open tracker is a manually placed 1 pixel beacon; its click tracker hides behind a subdomain of the brand's own website. Both are close to invisible unless you know what to look for.
Is Sailthru tracking your email? If a newsletter from a magazine, newspaper or large retailer is in your inbox, the odds are good. Sailthru email tracking runs on two mechanisms most recipients never notice: a transparent beacon dropped near the top of the template, and a redirect host that looks like it belongs to the brand rather than a marketing platform. The beacon logs the open. The redirect logs the click, then hands your identity to a cookie on the brand's site. Here is how each works, how to spot Sailthru in a message you already have, and how to stop both inside Gmail.
Key Takeaways
- Sailthru's documentation states that when the beacon image loads, "Sailthru logs information about the user and their session."
- Sailthru click tracking runs through a subdomain of the sender's own domain, typically link.brand.com, which brands create by pointing a DNS CNAME record at cb.sailthru.com.
- Every tracked Sailthru link carries the path prefix /click/ followed by numeric campaign identifiers and a Base64 encoded copy of your destination URL.
- On 17 July 2026 Sailthru shipped a pixel_tracking_consent profile field, and when the Pixel Tracking Compliance flag is on, profiles with no recorded preference are treated as opt out.
- Sailthru's Personalization Engine sets a sailthru_bid cookie when a subscriber clicks an email link, attributing later browsing on the brand's site to that campaign.
Does Sailthru Track Your Email?
Yes, and the documentation says so in plain language. Sailthru's tracking technologies page lists a beacon as "an image you include in your emails that Sailthru uses to track which users open your emails," and adds that "Sailthru will drop tracking cookies for your users to track engagement."
One structural detail separates Sailthru from most competitors: the beacon is not injected automatically at send time. A marketer has to place it. Sailthru's guide to tracking opens with a beacon tells customers "you will still need to ensure that the Zephyr code to display the beacon is included in all email templates." Every commercial template includes it, because open rate is the number the industry reports on. But a Sailthru email without open tracking is technically possible, which is not true of platforms that stamp the pixel on at send time.
What Is the Sailthru Beacon?
The Sailthru beacon is a single tracking image, unique to the sending account, inserted through a Zephyr macro rather than pasted in as a raw URL. The docs list {beacon}, {beacon_src} and their HTTPS variants, and recommend "a transparent 1-pixel by 1-pixel gif or png."
Placement instructions give away what it is for. Sailthru tells template authors to put the beacon "on line 1 of your template (before any HTML elements) or just after the body tag," and warns against the <head> section. The reason is clipping: Gmail truncates long messages, and a beacon below the clip line never loads. The tracker sits ahead of the content you came for. Our explainer on email open tracking versus click tracking covers the general mechanics.
The most useful sentence in the document is an admission: recipients who read mail without loading images "cannot be tracked unless they click a link in a message." That is the platform confirming that blocking images defeats it.
How Does link.brand.com Click Tracking Work?
Sailthru rewrites every link so it points at a subdomain the brand controls, then redirects through Sailthru before delivering you to the real page. Sailthru's account settings documentation describes the setup: pick a subdomain such as link.test.com, and "you will have to set a CNAME record on your DNS to point that subdomain to cb.sailthru.com." That line explains why Sailthru is so hard to see. The hostname belongs to the publisher. The server answering is Sailthru's.
Sailthru's developer documentation on handling Sailthru links states that "all Sailthru links are prefixed by click" and gives the example https://link.varickandvandam.com/click/13071627.1/aHR0cHM....., which resolves to an ordinary product page. Two numeric identifiers sit in that path, and the trailing string is your destination URL in Base64. Our breakdown of email link wrapping and URL rewriting shows how widely the pattern has spread.
How Can You Tell If an Email Came Through Sailthru?
Check the DNS record behind the link domain, because that is the one signal a brand cannot disguise. Hover a link, note the hostname, and run a CNAME lookup with the dig utility documented by the Internet Systems Consortium: dig CNAME link.example.com. If the answer is cb.sailthru.com, that newsletter is a Sailthru send, whatever logo is at the top.
Two softer checks need no terminal. A /click/ segment followed by digits and a Base64 blob is the Sailthru signature. Or open the message in Gmail on desktop, use the three dot menu, choose "Show original," and search for sailthru or sail-horizon. The From address tells you nothing, because Sailthru authenticates the brand's own sending domain. Our guide to how to tell if your email is being tracked applies the same inspection elsewhere.
What Happens After You Click a Sailthru Link?
The tracking moves from the inbox to the website, using cookies. Sailthru's Personalization Engine documentation describes a script served from ak.sail-horizon.com that sets a family of cookies on the brand's site: sailthru_hid to identify a known user and follow their activity, sailthru_content to record recent pageviews, and sailthru_bid, set "when a subscriber opens and clicks an email link, to attribute onsite actions to a particular email campaign."
Read that chain end to end. The beacon establishes that you read the newsletter. The redirect establishes that you clicked. The cookie ties everything you browse afterwards back to the email that sent you, in a profile keyed to your address. Interest tags accumulate from the pages you view, which is how the next send can be about the one topic you lingered on.
Can You Opt Out of Sailthru Open Tracking?
Only if the brand sending to you switched the feature on, and it is very new. Sailthru's release notes for 17 July 2026 introduced a pixel_tracking_consent field storing opt in, opt out or null. With the Pixel Tracking Compliance flag enabled, "Sailthru automatically suppresses the email tracking pixel for recipients whose pixel_tracking_consent value is opt-out," and null profiles count as opt out. That is a real privacy default, better than most of the industry offers.
The timing is the part nobody has pointed out. France's CNIL required brands to notify subscribers acquired before April 2026 and give them an opt out by 14 July 2026, per Zeta's summary of the CNIL restrictions. The consent field shipped three days later, after a decade of open tracking without one. We covered the CNIL recommendation and its 90 day clock when it landed.
Two limits matter from your side. It is a feature flag, off until a brand turns it on, and you cannot check whether yours did. And it covers the pixel only: nothing in that release touches the /click/ redirect or the sailthru_bid cookie, so an opt out recipient is still followed the moment they click. Regulators wrote rules about pixels. The industry still has the redirect.
Does Apple Mail Privacy Protection Stop Sailthru?
No, and Sailthru built a product line because of it. Apple's proxy fetches remote images before you see the message, so the beacon fires whether or not you read anything. Sailthru's answer is Real Activity Detection, which separates real opens from pre cached ones and produces a projected open rate; since August 2024 marketers can also segment audiences by real versus non human activity. The pixel is not blocked. It is reclassified.
Apple Mail users get their IP and read timing masked, at the price of an open logged against nearly every delivered message, which is why Apple Mail now fabricates roughly half of all recorded email opens. Gmail users get less than they assume: Google proxies remote images, hiding your raw IP, but the request still happens and the open still registers. Proxying is not blocking.
How to Block Sailthru Email Tracking in Gmail
Two things need stopping, and most tools handle only one: the beacon must not load, and the /click/ redirect must be bypassed so the request never reaches cb.sailthru.com. Gblock is a Gmail extension that blocks tracking pixels before they load and strips tracking redirects out of links, with a blocklist that updates automatically. The automatic part matters more here than with most platforms, because Sailthru's tracking hostnames are subdomains of thousands of separate brand domains rather than one fixed vendor host. A static blocklist cannot keep up with a new link.somepublisher.com every week; a maintained one can.
The alternatives deserve a fair hearing. Ugly Email and PixelBlock are free Gmail extensions that flag or block open pixels, and both do that job well; neither rewrites links, so the redirect and the sailthru_bid cookie survive. Trocker covers pixels across several browsers with the same gap. AdGuard filters system wide, though brand owned subdomains are where host based filtering struggles. Our comparison of email tracker blocker extensions sets the tradeoffs side by side.
Two manual habits work regardless of tooling. In Gmail, go to Settings, General, Images and choose "Ask before displaying external images," which Sailthru's own docs concede defeats the beacon, at the cost of breaking legitimate images everywhere. And when you follow a newsletter link, decode the Base64 portion and visit the destination directly. Both work and both are tedious, which is why a passive blocker is what people keep using. See how to block email tracking in Gmail for the full range.
Why Your Inbox Should Care About One Vendor
Sailthru is now owned by Zeta Global, which also runs a consumer identity graph. The signals a beacon collects are the same category such a graph is built from, which changes what an open means. We covered that ownership question in our piece on Zeta Global email tracking: this page is about the mechanics inside the message, that one about who holds the data.
The practical reason to care is volume. A daily newsletter is roughly 365 logged reading sessions a year from one sender, and Sailthru's customers skew heavily toward publishers. Multiply that across a normal subscription list and the beacon has a better record of your morning routine than your calendar does. None of it is negotiable at send time: the brand decides whether the consent flag is on and which domain the redirect uses. The one decision that is yours is whether the request leaves your browser at all.
Sources: Sailthru: Track Opens with a Beacon; Sailthru: Tracking Technologies; Sailthru: Account Settings and link domains; Marigold SDK Developers: Handling Sailthru Links; Sailthru: Personalization Engine Setup; Sailthru release notes, 17 July 2026.