If you inspect the HTML of a Buttondown email, you may notice a tiny, invisible link near the end — something like /api/emails/canary/.... That's a canary link. It is not click tracking or open tracking, and it is not used to measure subscriber engagement.
Why they exist
Many email providers and security tools automatically fetch every link in a message to scan for malware. That behavior is good for safety, but it causes real problems if we treat those fetches like human actions:
- Fake "clicks" that inflate analytics
- Accidental unsubscribes when a scanner hits a one-click unsubscribe URL
- Bot signups that get auto-confirmed when a scanner hits the confirmation link
The canary is a trap door only those scanners tend to walk through. When it trips, Buttondown knows the same machine's other recent link hits are automated and should be ignored — not counted as engagement, and not allowed to confirm a subscription or unsubscribe someone.
Why they stay when tracking is off
Click and open tracking are opt-in and only affect analytics. Canary links protect side effects that still exist when tracking is disabled: subscription confirmation, unsubscribe, and survey responses.
Turning tracking off means we stop rewriting your links and stop injecting a tracking pixel. However, mail scanners will still prefetch URLs, so the canary stays.
This is not subscriber tracking
A canary hit is never attributed as "this person opened/clicked your email." Humans don't interact with the link. We use the signal only to discard or block bot activity. No engagement score, open rate, or click rate is derived from canary visits.