I'm a huge proponent of domain modeling--that is, modeling behavior by the recipient's e-mail domain (hotmail.com, yahoo.com, aol.com). There are good reasons for this strategy. Consumers are becoming more advanced in their online behavior with broadband--they process things faster, and they have multiple e-mail accounts (2.7 on average) and typically have different e-mail personas. Add to that the fact that each ISP's interface is littered with obstacles to gaining a measured response (view or click); image blocking, filtering, and preview methods.
So, if they all represent different challenges, why should we look at them all the same way? E-mail clients are not all created equal, like browsers--and it's not as easy as disabling a pop-up blocker. Here are a few ISPs and basic descriptions of these obstacles.
America Online represents over 20 million subscribers, with the majority using version 8.0 or higher. AOL can represent almost 20 to 30 percent of consumer e-mail databases. In default mode, AOL will only render roughly a 350x350 pixel view before users have to maximize their screen. On top of that, image blocking is a standard setting--unless you are a trusted sender or part of the receiver's address book. If you are unlucky enough to get filtered to the spam folder, image blocking and links are disabled, and recipients must click on "Show Images" at the top of the window. Realistically, it could take up to two clicks (one to open the message and one to show images) before you even get a measured "open," and three to four clicks to get a measured click. Unlike Outlook and GMail environments, where the alt tags can give you a description of the image or link in the absence of images, AOL, Yahoo! and Hotmail/MSN do not allow the viewing of alt tags, rather they render blank or grey areas.
Yahoo! now boasts 60 million e-mail accounts. While Yahoo! does not apply image blocking as a default mechanism, if turned on it can pose a problem for the marketer. A receiver will have to scroll to the very bottom of the screen to click on the "show HTML" link. Additionally, rather than the 350 x 350 view of AOL, Yahoo! will give you a full horizontal view (roughly up to 600-700 pixels wide), but will only give you roughly 200 pixels vertical. This means that a receiver will only see a small portion of the header before they have click to max.
Hotmail/MSN represents roughly 120 million e-mail accounts, according to the last report I saw. This environment represents another type of challenge. Not only does it support image blocking in the inbox for non-white-listed senders; if filtered to Junk Mail, the receiver will need to click on the "this is not junk" button and then work through a series of windows to recognize the sender--then go back in and re-open the mail in the inbox, or they can click on "Enable all message content."
Now are you beginning to see why e-mail providers stress the importance of getting into the inbox unfiltered? To gain a measured response from a non-white-listed consumer with call to actions below 200 pixels, you could potentially have up to four different actions needed and gain a measured response. So, does that one click tell it all? My approach is to put a weighted value on views (opens) and clicks by domains. Take this along with close monitoring of share of clicks/views by domain, and you'll begin to understand what is really happening and the level of loyalty of your e-mail audience by the environment in which they interact with your brand.
If all these environments are so different, why do we put equal value on response metrics? Continually challenge the value of your metrics, your approaches, and how you put value on response.