How are cookies handled by email clients?
Let's say I set a cookie on an image request in an email (so the cookie is set when the user views the images in the email). If the user then clicks through the email so their browser opens, will the browser have access to the cookie I just set?
Obviously, if the user has Outlook as their email client and Firefox as their default browser, the email cookie (if it exists) will not be accessible. But what if they use IE? On an iPhone or Android phone, what happens?
Answer
I think you answered your own question...
It's a client-side problem (for you). It's "catch as catch can". No Guarantees any combination you use will be 100% fool-proof.
Outlook to Safari (or FF, Opera, etc). There are too many variables. Not to mention any one of your recipients could have their email client set to Plain Text.
I got the best tracking by assigning a query parameter to the images as well as ALL the links. But I've never found a solution that actually accounted for 100% of the emails that were sent (comparing/tracking view rates afterward).