[jcranmer]

Tracking market share of email clients is very difficult. To capture browsers, all you have to do is beg server logs off of the largest websites, and you can derive statistics for your own user base by crawling your own server logs.

But email? The usual way statistics are collected are by embedding 1×1 tracking pixels in email messages and noting who requests those images. But that will undercount any email client that has the option to refuse to load remote images, especially any one that turns that option on by default (such as Thunderbird). Another way is to look at the headers of large corpora of email, but the only ones that are publicly available are going to be mailing list archives, which will tend to have a more skewed distribution.