Created attachment 147503 [details]
mail with UTF-8 character in ASCII header
When a mail with some non-ASCII characters is encountered with the encoding of the header being (effectively) ASCII, KMail tries to decode it assuming ISO-8859-1. Because currently such occurrences tend to actually be UTF-8, this results in failed decodings in general. (I assume it was different in the past, when ISO-8859-1 was the most common ASCII-extension encoding used.) Obviously, the mail is malformed, because of the missing encoding information. So the strict thing to do is to show explicitly that the character(s) cannot be decoded, e.g., using the replacement character �. The lenient thing to do, nowadays, is to assume UTF-8.
STEPS TO REPRODUCE
1. List folder with mail with non-encoded non-ISO-8895-1-character in header
Observe that character is decoded as if it were ISO-8859-1
Observe that character is decoded as replacement character � or as if it were UTF-8.
KDE Plasma Version: 5.23.5
KDE Frameworks Version: 5.90.0
Qt Version: 5.15.2 (with https://community.kde.org/Qt5PatchCollection)