Why does reply tracking or open tracking sometimes hurt?

Tracking mechanisms modify your email in ways that some filters find suspicious. Open tracking requires embedding a tiny image that loads from an external server. Click tracking rewrites URLs to pass through tracking servers.

Some security-focused filters, especially in corporate environments, strip or block external tracking resources. They view any external call as potential information leakage or a tracking vector.

URL rewriting for click tracking changes your links from simple branded URLs to complex tracking URLs. This can trigger link reputation concerns if the tracking domain is shared with other senders who have damaged its reputation.

The tracking domain's reputation matters significantly. Using your own custom tracking domain rather than a shared ESP domain gives you control over that reputation.

Surveillance mechanisms draw attention. Balance your need for data with the filtering cost of collecting it.