The email client a person is using can only be detected if images are displayed. This can give an inflated weighting to email clients that display images by default, such as Outlook 2000 and the iPhone. It will also provide a lesser weighting to those that block images by default such as Gmail and Outlook 2007. Those email clients that aren't capable of displaying images, such as older Blackberry models and other mobile devices cannot be included in this study.
I've found tonnes of flaws in email clients where they fetch remote content even when they're configured not to, or the "Fetch Images" button hasn't been pressed.
An interesting tit bit of information: The basic (non IE) version of Outlook Web Access for Exchange 2007 blocks <img src="blah"> by default, but it actually fetches <input type="image" src="blah">
I've found a couple of similar issues in iOS and Apple Mail in the past which have now been fixed. Even if you disabled loading remote images by default, it would still fetch the contents of <audio> and <video> tags. This was fixed after I reported it. Then later, it started performing DNS prefetches before clicking "Load Images" if you included a tag like this: <link rel="dns-prefetch" href="http://blah/>;
Thunderbird had the dns prefetching flaw too. I reported that one and it was fixed.
Android had multiple problems last time I checked. It would even honour meta refresh tags and bounce you off to the web browser automatically, just from opening an email. Scary. I might give that another test soon.
I found similar flaws in Roundcube and OpenWebmail which were subsequently fixed after I reported them.
I built an automated testing tool a while ago which sends a specially crafted email to an address of your choice which includes lots of tricks to try and get your email client to call back to my server. You can check it out here:
I haven't found any issues with the gmail, hotmail or yahoo web interfaces, so there's no point testing those clients using my interface.
Gmail used to display SVG attachments which contained JavaScript, letting an email sender completely take over your email client. Someone else reported that and they fixed it.
They also don't share the methodology--for example, Entourage 2008+ and Outlook for Mac both use WebKit and all HTTP requests from them don't identify them as a client. Not sure if they are lumping that under Apple Mail or not accounting for it.
Email is the original locked-in platform (can't take your email address with you when you leave), and there is less and less incentive to switch now that so many people are reading their email on phone clients where every service looks the same.
If Ries and Trout ever publish a new edition of Positioning they should make this a case study. Moving email addresses is very painful for a lot of people, so no matter how much buzz Gmail gets in the tech community, there's still the built-in inertia to switching clients.
From the site: "Each time a subscriber opens an email sent with Campaign Monitor, we keep track of which email client they're using"
Maybe hotmail and yahoo users are more likely to open borderline spam whereas gmail users like to use email as more of a tool to get things done? Be sure to always read between the line before making strong inferences from pretty pie charts.
Surprising, really? Two examples: Outlook has been around for awhile and will continue to do so. The other example, Sparrow is just a few months (?) old and is quite popular.
If you're interested in stats, Campaign Monitor posts a monthly report on email client popularity: http://www.campaignmonitor.com/stats/email-clients/
Also note the Movers and Shakers stats at the bottom.