There are a lot of mail user agents out there.
That list of MUAs is also a tribute to the dedication of the guy who maintains the Display Mail User Agent extension for Thunderbird. His current stats:
Current state of work is:
5936 listed headers, 3818 matched headers (13 FULL, 71 PRE, 3622 A-Z, 112 POST), 3818 headers with icon (64.3% done), 3727 headers with URL (62.8% done)
That is really quite something. The extension is very cool; if a friend or family member emails saying “this thing about my mail doesn’t work”, you don’t have to go grovelling through their headers to see what MUA they are using. Also, if they are using Outlook or Outlook Express, you know they are on Windows, which helps.
(Don’t go and try the addon out straight away if you are using Shredder, though – the reason I was investigating the website was that I was about to report a bug that the header pane disappears entirely on Shredder with the latest version of DispMUA.)