Why is read tracking not working with distribution lists and Google Groups?

Email tracking works by inserting different transparent images for each recipient in a sent email.

e.g. TO: a@acme.com, b@acme.com

This email would be sent twice with identical versions apart from the distinct transparent pixel for each recipient.

This way when a@acme.com opens the email in their email client, the email client requests the transparent image from our server. Knowing that this image was used only in the version of the email sent to a@acme.com, we can safely say that a@acme.com read the email.

The problem with distribution lists is that the pixel will be the same for all recipients.

e.g. TO: distribution@acme.com

This email would be sent once with one transparent pixel. The distribution list server will then forward that email to everyone on the list, including that same unique image. Thus, whenever someone from the list reads an email, their email client will load that same image.

We have no way to know that those emails were open by different people.

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