Symptom: links work in tests but fail in sent mail with invalid-signature or expired-token errors.

1. Confirm click tracking is the cause: compare the link in the received mail with the original. A sendgrid.net redirect wrapper means the signature no longer matches.
2. Disable click tracking for the message via tracking_settings.click_tracking.enable false in the v3 API, or the clicktrack filter in X-SMTPAPI for SMTP.
3. For a single link inside an otherwise tracked mail, add the clicktracking=false attribute to that anchor tag. The doc set supports per-link opt-out.
4. Do not disable tracking globally to fix one flow. You lose engagement data everywhere for a local problem.
5. After disabling, resend and verify the raw link survives byte-identical.

Rule: any URL carrying a cryptographic signature or single-use token goes out untracked.