What is an image proxy?
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An image proxy is a server that downloads email images before showing them to the recipient. Instead of loading images directly from the sender's server, the email client (like Gmail, Yahoo Mail, or Outlook) fetches them first, caches them on its own server, then displays the cached version to the user.
Why do email clients do this? Privacy and security. When an image loads directly from the sender's server, the sender can see the recipient's IP address, location, device type, and exact time the email was opened. Image proxies break that tracking. The sender only sees the proxy server's IP (Google's or Yahoo's), not the individual user's.
For senders, this changes how open tracking works. Most ESPs track opens by embedding a tiny invisible pixel in every email. When that pixel loads, they count it as an open. But with image proxies, the proxy downloads that pixel immediately (often before the user even sees the email), which inflates open rates. Or the proxy caches the pixel once and serves it to every user without pinging your server again, which deflates open rates. Either way, open rates become less reliable.
Image proxies also affect deliverability signals. Mailbox providers like Gmail used to look at whether users loaded images as a sign of engagement. Now that Gmail's proxy loads images automatically, that signal is gone. What still matters: clicks, replies, stars, and whether the user moves the email to a folder.
Which email clients use image proxies? Gmail (both consumer and Google Workspace), Yahoo Mail, AOL Mail, and Outlook.com all proxy images. Apple Mail introduced Mail Privacy Protection in 2021, which works similarly but also blocks the sender from seeing whether the user opened the email at all. Desktop email clients like Outlook and Thunderbird don't proxy by default (they load images directly or block them entirely, depending on user settings).
If you're tracking engagement, don't rely on open rates alone. Focus on clicks, replies, and conversions. Those metrics aren't affected by image proxies and they're what mailbox providers actually care about.
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