Information from developers:
Root cause: new Courier-IMAP 5 implements
rfc6855 and its developers opted to ALERT any IMAP client that attempts to fetch a mail message with national symbols (UTF-8) in headers, but didn't explicitly enable UTF-8 support. Outlook does exactly this.
How to fix: there is no fix available. Outlook doesn't seem to support rfc6855 at all. My recommendation so far is to do either of:
- Just deal with it (messages are annoying but the messages themselves are displayed correctly);
- Use another mail client, e.g. webmail.
Internal report PPPM-11248 was submitted.