I've been stuck 31.7 for years because it's the only version which honours font preferences in OS X. Theme and Font Size Changer was useless after version 41.1. Someone please tell they've fixed this nonsense.
I read that as "I've been stuck 31.7 years because it's the only ..." and did a triple-take on how old the software is and how long you've been using it. But no, Wikipedia lists it as "Initial release: July 28, 2003; 14 years ago".
Still very old to be in such widespread use, though. From where I worked to my grandma to my in-laws to the Linux poweruser that I am, a very widespread range of people use it. I am not 100% happy with it, but email is important enough that I want something stable (e.g. no bugs that either mess up my email server-side or stop it from working) and secure (both the connection and for viewing).