[ralph84]

Indeed, the three email tracking services I'm familiar with (HubSpot, Yesware, and Outreach) all generate unique URLs per recipient. The only information you don't get for Gmail clients is location, but you get the most valuable info which is if/when/how many times the email is opened.

[neeleshs]

I remember reading that gmail caches these images, rendering the how-many-times useless.

[alaties]

GMail absolutely caches images. The cache is primarily for performant delivery of data over mobile networks though and reduced load of repeat same-day viewing, and less for privacy.

While their cache is large, it's not infinite. If what you're interested in is whether a customer engages with an email multiple times over a few days, you'll likely get the pixel hits to confirm it over that time period. Of course, as you imply, you wouldn't be able to collect how many times an hour a single customer has viewed an email sent to them.

[godmode2019]

I have tested this and the cache expires pretty quickly. From memory 10min -30. I thought that was fine because its around about one hit per session.

You get enough good information. The IP address would be even better however.