How do image proxies affect open tracking?
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Image proxies change what information your ESP gets when a tracking pixel fires, and in some cases change when it fires.
When a recipient opens your email through a proxied email client, the image request comes from the proxy server's IP, not the recipient's. Your ESP records an open, but the metadata (IP address, location, device info) belongs to Google or Apple, not your subscriber. Geographic and device reports become less accurate. If you're making decisions based on the "where are my subscribers" or "what devices are they using" data, treat those numbers with skepticism.
For timing: if Gmail caches the image (which it does after the first load), subsequent opens of the same email by the same user may not generate new pixel requests. Your re-open count can be understated for Gmail recipients.
For Apple Mail with MPP enabled, the proxy pre-fetches images shortly after delivery, before the user opens. Your ESP records an open from Apple's servers. The user may never actually read the email. This is what inflates open rates for senders with heavy Apple Mail audiences.
The practical guidance: don't make campaign decisions based on open time data or geographic data without filtering out proxy IPs first. Use click rate as your primary engagement signal where possible. Clicks require a human to actually tap or click a link, which proxies don't do automatically.
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