Sorry, Swedish is absolutely not "more special" than any other language with "weird characters" (and yes, I'm including my own native Russian here).
If by "message comes from web" you mean messages sent by some script / automated service -- it's very common for those to omit the "encoding" for subject and / or message text.
Aqua has code to try and guess, but its abilities are limited -- esp. considering that sending non-English characters without an encoding is not valid at all.
Despite libraries available for sending mail for any programming language, this is common situation. The *right* thing to do is fix the sending script.
Now if you wish, you can send me (kmansoft / gmail) the original source of a sample message, so I could take a look. Either grab it in web mail ("show original", "view source", etc.) save to a .txt file (no Word please) and send as an attachment...
Or use "forward as attached .eml" in Outlook (not regular "forward" where you can edit the original text).
