[mattbeckman]

Once you open the email, a tracking pixel will fire (usually an <img> tag), which is far more useful to the email marketer.

Unsubscribe is your best bet as honoring opt-outs are protected by the CAN-SPAM act.

[hu3]

Gmail doesn't load images until I athorize:

https://i.imgur.com/RZ93VIU.png

[BenjiWiebe]

Thunderbird doesn't either.