Unless you manually change the encoding settings, KMail will send messages using US-ASCII if possible, then ISO-8859-1 and will only use UTF-8 when using ISO-8859-1 is not possible. Since UTF-8 has been a de facto standard for years, the use of ISO-8859-1 sometimes causes a few problems. For example, I regularly send messages to a mailing list. The list server automatically adds a few lines of text at the bottom of my messages, using UTF-8 encoding. The result is that those added lines appear broken, because their encoding does not match the one of my messages, which is ISO-8859-1. I cannot see why it would be better to keep using ISO-8859-1 instead of UTF-8, so I think KMail should now default to UTF-8. This is what most other mail clients do.
> The list server automatically adds a few lines of text at the bottom of my messages, using UTF-8 encoding. Not respecting the encoding of the message is a bug that you should report to the admins of that list server.