Definition
A link alias (also called a tracking link, click link, or rewrite URL) is a URL that an email service provider (ESP) substitutes for the original destination URL in an email. When a subscriber clicks a link, the ESP's server registers the click event first, then redirects the subscriber to the original destination. This is how open-rate tracking pixels and click-tracking work in most email marketing platforms.
Link aliases typically follow a format like https://email.example.com/t/12345/abcde/ where the path contains encoded identifiers for the campaign, subscriber, and link. The alias is transparent to the subscriber — they see a preview of the final URL in their browser's status bar, and the redirect happens in milliseconds, but the actual click path goes through the tracking server.
How Link Aliasing Works
When you add a link to an email campaign in most ESPs, the platform automatically rewrites the href attribute at send time. The original URL is stored in the ESP's database, and the alias URL is generated dynamically per subscriber per campaign. Each alias is unique, which allows the ESP to identify exactly which subscriber clicked which link in which campaign.
The alias URL contains encoded data: a campaign identifier, a subscriber identifier, and a link identifier. This data is usually obfuscated but not encrypted — it is designed for routing and analytics rather than security.
Best Practices
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Ensure link aliases resolve to the correct destination: Broken redirects happen when ESP configuration or campaign setup is incorrect. Always test all links in a test send before campaign launch.
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Be aware of alias URL length: Some alias URLs can be very long, especially for ESPs that include extensive tracking parameters. Long URLs can break in some email clients, particularly Outlook's 2,083 character limit.
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Use a custom tracking domain: Most ESPs allow you to set a branded tracking domain (e.g.,
links.yourdomain.com) instead of the ESP's default domain. This improves inbox recognition and deliverability. -
Check that aliases work with URL preview tools: Some link preview tools (like Slack, Twitter, and messaging apps) follow redirects. If you share your email's final URL in a channel that follows redirects, the click fingerprint may be consumed before the intended recipient sees it.
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Monitor alias redirect speed: Slow redirect servers delay the subscriber's experience and can lead to higher click-to-open latency. Ensure your ESP's tracking infrastructure has adequate capacity for your send volume.
Related Glossary Terms
A/B Testing
A/B testing in email marketing is the practice of sending two variations of an email to a small sample of your list to determine which version performs better before sending the winner to the remaining subscribers.
Abandoned Cart Email
An abandoned cart email is an automated message sent to customers who added items to their online shopping cart but left without completing the purchase. It is one of the highest-converting email types in ecommerce.
AMP for Email
AMP for Email is a Google-developed framework that allows email messages to include interactive elements like forms, carousels, accordions, and live content. It turns static emails into dynamic, interactive experiences directly inside the inbox.
Bounce Rate
Email bounce rate is the percentage of emails that were rejected by the receiving server before reaching the recipient. It is a key indicator of list health and data quality.
CAN-SPAM Act
The CAN-SPAM Act is a US law that sets rules for commercial email. It requires accurate subject lines, a physical address, a clear opt-out mechanism, and prompt processing of unsubscribes. Violations can result in penalties up to $51,744 per email.
Click-Through Rate
Click-through rate (CTR) is the percentage of email recipients who clicked one or more links in your email campaign. It measures how compelling your content and call-to-action are.
Frequently Asked Questions
Yes, but the effect is usually positive. Using a custom tracking domain aligned with your sending domain improves link reputation. ESP default domains (like sendgrid.net or mailchimp.com) may be treated with suspicion by mailbox providers because they appear unrelated to your brand. A custom tracking domain that matches your sending infrastructure improves trust signals.
Some email clients, particularly corporate Outlook environments and security gateways (Proofpoint, Mimecast), may rewrite or strip tracking links. This can break click tracking for those subscribers. Using HTTPS for all alias URLs and maintaining reputable tracking domains reduces the likelihood of blocking.
No. Link aliases are infrastructure-level URL rewrites for click tracking and attribution. UTM parameters are marketing-level query strings added to the final URL for analytics platform attribution (Google Analytics, etc.). Both can exist on the same link — the alias routes through the tracking server, and the UTM parameters appear in the final destination URL's query string after the redirect.
Most ESPs keep link aliases active indefinitely for historical reporting. However, changing your ESP or campaign structure may invalidate old alias URLs. If a subscriber tries to click a link from an email sent with your old ESP, the alias will return a 404 or redirect error.
If the tracking server is unreachable, the subscriber sees a timeout or error page instead of the destination URL. This is why tracking infrastructure should be monitored separately from sending infrastructure. Some ESPs include a fallback — the original URL may be embedded in the alias URL itself so it can be extracted if the redirect fails.