Is it correct to write rules in the following syntax:
if header :contains "Subject" "foreign characters, for example åäö" {
discard;
}
and store it in UTF-8
Yes, it is correct.
(åäö, are some swedish characters, for your information.)
Then it is a question for the sieve implementation to handle such a
script I suppose.
Yes. Implementations SHOULD support this; see section 2.7.2 of RFC 3028.
I tried it at CMU Sieve 2.0, but it seems not to be implemented.
That's unfortunate, but having implemented this I can sympathize; it is a
major pain to do.
Ned