[mongrel]

I’m guessing this is not a function of gmail itself. Streak probably inserts code that signals back whenever something is viewed. Constant contact and mail chimp already do that cross platform. Every email you’ve already received from mass emailers already tracks all this stuff.

[Windflier]

I’m guessing this is not a function of gmail itself. Streak probably inserts code that signals back whenever something is viewed.

Correct. It's an add on program.

[iontheball]

I’m guessing this is not a function of gmail itself.
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Getting a return receipt is not a new feature. I had it working with an Outlook email program used by the Federal government back in the 90s. I found it to be a helpful feature and would like to have on my email.

[Moonman62]

The way they used to do it was to embed a one pixel image in the email. The server storing the image can tell when the image is loaded.