Every email tracking link — the URL that records when a recipient clicks a link in an email — routes through a domain owned or operated by the tracking system. When an ESP routes tracking links through its own shared domain (links.mailchimpsend.com, click.sendgrid.net, track.klaviyoemail.com), the reputation of that shared domain affects how spam filters evaluate the links in emails. A shared tracking domain that is abused by other senders on the same platform can cause spam filter scoring problems for legitimate senders who have no control over who else uses the same tracking domain. Custom tracking domains — branded tracking subdomains that the sender controls — eliminate this shared domain risk and build link reputation under the sender's own brand.
Why Shared ESP Tracking Domains Hurt Deliverability
When an email contains links routed through a shared ESP tracking domain (click.sendgrid.net, for example), spam filters that evaluate link reputation check the reputation of click.sendgrid.net — not brand.com. If other senders on the same ESP use the platform's shared tracking domain to send spam, phishing, or other harmful content, the shared domain accumulates negative reputation signals that affect all senders using it.
The practical effect: a legitimate sender with excellent sending reputation and High Gmail domain reputation may still experience elevated spam filter scoring at corporate gateways (Proofpoint, Barracuda) or consumer spam filters because the links in their email route through a shared tracking domain that has been flagged for abuse by other senders on the same platform. The sending domain (brand.com) has good reputation; the link domain (click.sendgrid.net) has degraded reputation from co-tenant abuse; the combined evaluation of both produces higher spam filter scoring than either would alone.
URL shorteners in email (bit.ly, tinyurl.com) create an even more severe version of this problem — these domains are used so heavily for spam and phishing distribution that most corporate email security gateways automatically apply maximum suspicion scoring to any email containing URL shortener links, regardless of the sending domain's reputation. URL shorteners should never be used in commercial email.
How Email Click Tracking Works
Email click tracking intercepts the link click event to record it before redirecting to the actual destination. When an ESP generates a tracked link, it transforms the original destination URL into a tracking URL format:
# Original link in email template: https://www.brand.com/summer-sale # ESP transforms to tracked URL (shared domain): https://click.sendgrid.net/LS/BW4000/eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI... # With custom tracking domain (CNAME to ESP's servers): https://click.brand.com/LS/BW4000/eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI...
The tracking URL encodes the recipient identifier, campaign identifier, and original destination URL in the path. When the recipient clicks the link, their browser requests the tracking URL from the ESP's servers (via the CNAME that points click.brand.com to the ESP's tracking infrastructure). The ESP's server records the click event (which contact, which campaign, which link), then redirects the browser to the original destination URL. The recipient sees the original destination; the ESP records the click. The custom tracking domain (click.brand.com) makes this process route through the brand's own subdomain rather than the ESP's shared domain.
Setting Up a Custom Tracking Domain
Custom tracking domain setup requires two steps: configuring the DNS CNAME record that points the custom subdomain to the ESP's tracking servers, and configuring the ESP to use the custom domain for tracking links. The process is similar across all major ESPs:
Step 1 — Choose the tracking subdomain: Select a subdomain that is clearly associated with the sending brand but distinct from the primary domain. Common formats: click.brand.com, links.brand.com, track.brand.com, e.brand.com. The subdomain should not be the same as any subdomain used for the website, API, or other services — tracking domain traffic behaves differently from web traffic and should be isolated on its own subdomain.
Step 2 — Add CNAME record in DNS: Publish a CNAME record in the DNS zone for brand.com that points the tracking subdomain to the ESP's tracking server hostname. The target hostname is provided by the ESP's custom domain setup instructions:
# Example CNAME records for common ESPs: # (use the hostname provided by each ESP — these vary) click.brand.com CNAME sendgrid.net. ; SendGrid click.brand.com CNAME pages.awsapps.com. ; AWS SES click.brand.com CNAME mailchimp.com. ; Mailchimp click.brand.com CNAME klaviyo.com. ; Klaviyo # TTL recommendation: 300 seconds (5 minutes) # Low TTL allows faster updates if the ESP's target hostname changes
Step 3 — Configure the ESP to use the custom domain: Each ESP has a settings section for custom tracking domains. Upload or enter the custom domain subdomain (click.brand.com) and the ESP validates it by checking that the CNAME record resolves correctly to its tracking servers. After validation, the ESP begins using click.brand.com instead of its shared tracking domain for all new campaigns sent from the account.
Step 4 — Verify tracking works: Send a test campaign, click a link, and confirm the URL in the browser address bar shows click.brand.com (not the ESP's shared domain) before redirecting to the destination. Check the click tracking data in the ESP's campaign analytics to confirm the click was recorded correctly.
SSL/TLS Certificate Requirements
Custom tracking domains must serve over HTTPS — HTTP-only tracking links trigger browser security warnings in modern browsers and are increasingly flagged by spam filters as suspicious. The SSL certificate for the tracking subdomain is typically provisioned automatically by the ESP as part of the custom domain setup — the CNAME configuration routes the subdomain to the ESP's infrastructure, and the ESP handles SSL certificate provisioning for the custom domain using Let's Encrypt or similar automated certificate management.
Verify SSL is active for the custom tracking domain: navigate to https://click.brand.com in a browser. It should return a valid SSL certificate (the padlock icon should appear; no certificate warning). The certificate should be issued to click.brand.com or *.brand.com (wildcard). If SSL is not working, check that the CNAME record is correctly configured and that the ESP has completed its domain validation process — most ESPs require the CNAME to be visible in DNS before they provision the SSL certificate for the custom domain.
Tracking Domain Subdomain Strategy
For programmes with multiple sending streams (transactional and marketing from the same domain), using separate tracking subdomains per stream maintains stream isolation at the link reputation level as well as the sending IP level:
click.brand.com— marketing campaignslinks.brand.comort.brand.com— transactional email (order confirmations, account alerts)eclick.brand.com— e-commerce specific
This stream separation means a tracking domain reputation event from marketing campaign links (which have higher spam filter scrutiny from higher volume and promotional content) does not affect the link reputation of transactional tracking domains — which are critical for reliable delivery of operationally important email. The same isolation principle that applies to sending IPs and sending domains applies to tracking domains: separate streams, separate reputation exposure.
For agencies or multi-client senders, using per-client tracking subdomains (click.clientbrand.com rather than click.agencyname.com) provides client brand alignment and prevents any single client's link reputation from affecting other clients' tracking domains. This requires separate DNS configuration for each client's domain, which adds management overhead but provides complete isolation.
Configuring Custom Tracking in Major ESPs
SendGrid: Settings → Tracking → Custom Link Branding. Add subdomain, add the provided CNAME record to DNS, click Verify. SendGrid provisions SSL automatically once CNAME is visible.
Mailchimp: Account → Domains → Add and Verify a Domain. Mailchimp uses a CNAME-based verification. After verification, go to Campaigns → Tracking and select the custom domain.
Klaviyo: Account → Settings → Email Domains → Tracking Subdomain. Add the subdomain, copy the CNAME value to DNS, verify. Klaviyo uses the custom domain for both click and open tracking.
Postmark: Servers → Default Server → Sender Signatures → Edit → Custom Tracking Domain. Add subdomain, add CNAME to DNS, save. Postmark handles SSL provisioning automatically.
Brevo (Sendinblue): Settings → Domains → Tracking Domain. Add subdomain, configure the CNAME record provided, verify. Available on paid plans.
Monitoring Tracking Domain Health
Custom tracking domains require the same monitoring as other email infrastructure DNS records. The tracking domain CNAME must remain correctly configured — if the CNAME record is modified or deleted, click tracking links in previously sent campaigns stop working (recipients who click links in archived or forwarded email get an error), and new campaigns may fail to generate tracking data. Include tracking domain CNAME records in the DNS monitoring infrastructure alongside SPF, DKIM, and DMARC records.
Blacklist monitoring for tracking domains: check click.brand.com (not just the IP addresses) against URL blocklists (Surbl, URIBL, Google Safe Browsing) periodically. A tracking domain that appears on a URL blocklist causes email containing tracking links to score higher on spam filter content evaluation. MXToolbox provides domain-level URL blocklist checking in its reputation monitoring tools. Any URL blocklist listing for a tracking domain requires immediate investigation — typically caused by spam complaints that included the tracked URL or by a compromised campaign that distributed spam or phishing links through the tracking infrastructure.
Tracking Domains and Spam Filter Evaluation
Custom tracking domains improve spam filter content scoring primarily by replacing a widely-shared domain (used by thousands of other senders on the same ESP) with a domain used exclusively by the sending programme. The link reputation of click.brand.com reflects only the programme's own sending history — not the aggregated history of thousands of co-tenants on the shared tracking domain.
The content scoring improvement from custom tracking domains is most pronounced at corporate email security gateways (Proofpoint, Barracuda) that maintain URL reputation databases. These gateways apply URL reputation scoring to each link in an email as part of their content evaluation — a tracking domain with a clean, single-sender history scores better than a shared domain with mixed-quality history. For programmes sending to corporate environments (B2B email, financial services, healthcare), the custom tracking domain benefit is more commercially significant than for programmes sending primarily to consumer ISPs like Gmail and Yahoo, which weight URL reputation less heavily relative to domain and IP reputation.
Custom tracking domain setup is a one-time configuration task that produces permanent deliverability benefits. The CNAME is published, the ESP is configured, and the tracking domain reputation builds automatically with every campaign send thereafter. There is no ongoing operational cost beyond the periodic monitoring described above. For any programme that has not yet configured a custom tracking domain, it is the highest-ROI single configuration task available — taking less than 30 minutes to implement and providing permanent link reputation independence from the ESP's shared tracking infrastructure.
Custom Tracking Domain vs No Tracking
Some email programmes — particularly cold email, where tracking is associated with surveillance concerns among recipients, and plain-text-focused newsletters — operate without any click tracking. No-tracking sending eliminates the tracking domain reputation variable entirely: all links are bare destination URLs that go directly to the target without routing through any tracking intermediary. This approach has genuine merits for audiences that are suspicious of tracking and for programmes where reply rate is more valuable than click rate data.
For programmes that need click tracking data (e-commerce attribution, A/B test evaluation, automation triggers based on link clicks), the custom tracking domain is non-negotiable — it is the only way to maintain tracking functionality while building link reputation under the programme's own brand rather than sharing reputation exposure with thousands of other ESP users. The choice is not "custom tracking domain vs shared tracking domain" — it is "custom tracking domain vs no tracking" — and the programmes that need click data should choose custom tracking domains as the least-harmful way to implement it.
Custom tracking domain configuration is one of those infrastructure details that separates programmes that have been deliberately optimised from those that use ESP default settings. Default settings are designed for onboarding simplicity, not for production deliverability performance. The custom tracking domain, the custom DKIM signing domain, the per-ISP concurrency configuration — each of these moves the programme away from shared ESP defaults and toward a configuration that reflects the programme's specific requirements and builds reputation under the programme's own brand. That accumulation of deliberate configuration choices is what makes a programme's deliverability self-reinforcing rather than dependent on the ESP's average pool quality.