Seguimiento de correo electrónico por destinatario: cómo funcionan los enlaces de identidad
A marketing email lands in your inbox. The subject line is generic. The greeting says "Hi there." You delete it without a second thought. But another email from the same sender addresses you by name, references your recent download, and includes a link that seems tailored to your specific interest. You click. That second email worked because the sender used per-recipient email tracking—specifically, identity links—to personalise the experience and measure engagement at the individual level.
Per-recipient email tracking is the mechanism that lets you know exactly who clicked what, and when. The most reliable way to achieve this without breaking deliverability or privacy expectations is through identity links. Here is how they actually work, what makes them different from generic tracking, and why you should care about the implementation details.
What identity links are and why they matter for tracking
An identity link is a URL that contains a unique, non-guessable identifier tied to a single recipient. When that recipient clicks the link, the identifier is decoded by your tracking server, and the event is logged against their profile. This is the core of per-recipient email tracking.
Without identity links, you only get aggregate data. You know that 12 people clicked a link, but you have no idea which 12. With identity links, you can answer questions like: Did the VP of Engineering click the demo link? Did the lead from last week's webinar open the case study? Did the recruiter who ghosted you finally view your portfolio?
The identifier itself is typically a hash or a UUID appended as a query parameter or embedded in the URL path. For example:
https://yourdomain.com/resource?uid=a1b2c3d4e5f6
This identifier is meaningless to anyone who intercepts it, but your server can map it back to the recipient's email address or CRM record. The key constraint is that the identifier must be unique per recipient per campaign, or at least per recipient per link, to prevent false positives.
How identity links differ from pixel-based tracking
Many email trackers rely on a tracking pixel—a transparent 1x1 image embedded in the HTML. When the recipient's email client loads the image, the pixel request includes a unique identifier, and the server logs an open event. Pixel tracking is simple, but it has two major weaknesses.
First, modern email clients block images by default. Gmail, Outlook, and Apple Mail all require the user to explicitly download images, or they proxy them through their own servers, which can strip tracking identifiers. This makes open rates unreliable. Second, pixels cannot track clicks. They only tell you that the email was opened, not which links the recipient engaged with.
Identity links solve both problems. They do not rely on image loading. They work as long as the recipient clicks a link, which is a deliberate action. Clicks are a stronger signal than opens. A click indicates intent. An open might just be an auto-preview pane or a mobile notification.
The trade-off is that identity links require you to generate unique URLs for every recipient. This adds complexity to your email sending pipeline, but the data quality is worth it for any campaign where per-recipient insight matters.
The mechanics of generating and resolving identity links
Generating identity links involves three steps: encoding, injection, and resolution.
Encoding
You need a way to map a recipient identifier to a unique token. The simplest approach is to base64-encode the recipient's email address or database ID, but this is trivially reversible. A better approach is to use a cryptographic hash with a secret salt, or a randomly generated UUID that you store in a lookup table.
For example, using a HMAC-SHA256 hash of the recipient ID combined with a campaign ID gives you a token that cannot be reversed without the secret key. This prevents a malicious actor from enumerating tokens to harvest recipient identities.
Injection
During email generation, your template engine replaces each link with a version that includes the unique token. This must happen at send time, not at template design time. If you use a merge tag like {{tracking_id}}, the engine substitutes the correct value for each recipient before the email is dispatched.
This step is where most implementation bugs occur. If you accidentally reuse the same token across multiple recipients, your per-recipient tracking collapses into per-campaign tracking. If you forget to URL-encode the token, the link may break in certain email clients.
Resolution
When the recipient clicks the link, the HTTP request arrives at your tracking server with the token in the query string. The server looks up the token in your database, retrieves the recipient's profile, logs the click event with a timestamp and user agent, and then issues a 302 redirect to the final destination URL.
The redirect is important. It ensures the recipient lands on the intended page without seeing a tracking intermediary. It also allows you to append UTM parameters dynamically based on the recipient's segment or campaign, which is how newmail.click handles attribution.
Privacy considerations and data protection
Per-recipient email tracking sits in a grey area of data privacy. You are collecting behavioural data tied to an individual's email address. This is personal data under GDPR, CCPA, and similar regulations. You must have a lawful basis for processing it, typically consent or legitimate interest, and you must disclose the tracking in your privacy policy.
Identity links are less intrusive than pixel tracking in one sense: they only fire when the user takes an active step. But they are more intrusive in another sense: they reveal exactly which recipient performed which action. If you share a link on social media or forward an email, the token is exposed. Anyone who clicks that forwarded link will be tracked under the original recipient's identity.
To mitigate this, you can implement a "forwarding guard" that detects mismatched user agents or IP addresses and anonymises the event. You can also set a short expiration on tokens so that old links stop tracking. Some platforms allow recipients to opt out of per-recipient tracking entirely, falling back to anonymous link tracking.
The safest approach is to be transparent. Include a one-sentence notice in your email footer: "Links in this email are individually tracked to improve our content. You can disable tracking in your preferences." This builds trust and keeps you compliant.
Why per-recipient tracking fails with common email clients
Even with perfect identity link implementation, per-recipient email tracking can break due to client-side behaviour. Here are the three most common failure modes.
- Link wrapping: Gmail and Outlook sometimes wrap links through their own proxy servers for security scanning. This can strip or modify your tracking parameters. The fix is to use HTTPS everywhere and ensure your tracking domain is whitelisted by major providers.
- Preview panes and link prefetching: Apple Mail and some desktop clients prefetch links in the background to speed up rendering. This generates a click event before the user actually clicks. You can mitigate this by ignoring events that arrive within 500ms of the email being opened, or by requiring a JavaScript-based confirmation on the landing page.
- Corporate firewalls and email security gateways: Many enterprises scan all outbound links and replace them with their own tracking URLs. This breaks your identity link entirely. There is no reliable workaround except to ask recipients to add your domain to their allowlist, which rarely happens.
The practical takeaway is that per-recipient email tracking gives you directional data, not absolute truth. You will see false positives from prefetching and false negatives from link rewriting. Treat the numbers as trends, not census counts.
How newmail.click handles this
Newmail.click is a free email signature builder, but we also offer a Pro tier with per-recipient click and open tracking. When you enable tracking on a campaign, we generate a unique identity token for each recipient using a salted HMAC hash. That token is injected into every link in your email signature or body. When a recipient clicks, we log the event, resolve the token to the recipient's profile, and send you a Slack or email alert in real time. We also append default UTM parameters so your analytics platform receives clean attribution data. The entire flow is transparent—you can see the token in the link preview, and we never share or sell the identity data.
The one metric that matters
Per-recipient email tracking through identity links gives you the ability to score leads, segment follow-ups, and measure content resonance at the individual level. It is the difference between knowing that a campaign performed well and knowing that a specific person took a specific action. Implement it correctly, and you stop guessing.