Åäö is replaced by � in Apple email app

Jonteemil wrote:

so the problem is only when viewing my emails in the Apple email app, and from one specific sender, namely noreply-se@klarna.se. Other emails render the åäö just fine.

This is a clear situation where the sender's device is creating this letter encoding.

If the issue occurs only for emails from specific senders, it is possible that the problem lies on their end. Reach out to them and ask if they are using the correct encoding when sending emails.