1x1 invisible images that are actually linked back to a PHP script (that sends appropriate content-type and readfile data back) is the traditional way... and then a redirect script for all outgoing links.

I'd love to hear if anyone has other ways.

There is no way to get particularly RELIABLE data for email results.

As owol mentioned, the usual method is including an special image in the message. The image link would be something like http://www.yoursite.com/image-stats/AAAA-BBBB.gif

AAAA would be a unique identifier for the subscriber, and BBBB would identify the message, so if that image gets requested then your server knows that AAAA has opened message BBBB and can log the "open" action.

The problem comes in that there are two very common things that can prevent that from happening. 1) If their email reader only looks at plain-text or 2) The email reader does not allow remote images to load. (in Gmail, for example, you have to specifically allow it for each message/sender)

So you can only measure a sub-set of your subscribers, and since you don't know the actual size of this sub-set, you really have no idea of your true open rate.

Again with deliverability... you can (MUST!) handle and record any bounce-backs to non-deliverable addresses, but you'll never know how many messages disappear into a black hole.

Unless I'm missing some magic bit, even the experts like Aweber can't overcome these fundamental problems.

-Ryan

Still thinking after clicking "post"...

The conclusion (in my mind) is to pay little to no attention to email open stats, and focus on the more valuable RESPONSE rates. When you link to your offers you can include the same unique subscriber-message style identifiers.

http://www.yoursite.com/AAAA-BBBB/click-here-for-my-amazing-offer

-Ryan