A tracking pixel is an invisible 1-by-1 image added to an email. When the recipient's email app loads it, the sender's server records an open. It's how every email tool measures open rates.
Why open rates are unreliable
- Apple Mail Privacy Protection loads every image on arrival, so Apple Mail users show as opened whether they read the email or not.
- Apps that block images never record an open, even when the email was read.
- Security scanners in company mail systems load images to check them.
The cost for cold email
A tracking image makes a plain email look more like marketing, and its address can hurt deliverability on a shared tracking domain. Many cold emailers switch open tracking off and judge campaigns by replies, the number that matters. mailzy leaves it off by default, and when it's on, opens made automatically by Apple Mail are counted separately, not as real opens.