[matt_heimer]

Same way as always, image loading. If image URLs are unique and external then you can track their opening. Not all users will allow it so you also track clicking on in email links.

[TylerE]

Doesn't just about every mail service proxy, and often preemptively fetch those?

[jeroenhd]

Not every service, buy many of them do.

[TylerE]

I'm quite sure GMail does... which is basically the entire internet as far as mail delivery.