Mail: foreign characters in subject wrong

[AmigaRoots]

The subject of email messages with foreign (Greek in this case) characters will not display correctly, I get something like: =?UTF-=CE=BF=CE=BD_=CE=9F=CE=A4=CE=9... etc. I've set the regional and locale settings to Greek but it doesn't help, and I can't find any specific settings for the Mail app. Is there a solution? Thank you.

update: it looks like this is sender-specific since some emails are displayed correctly.