Trojita should detect (arguably malicious) IMAP servers which do not support the UTF-8 charset and resort back to disabling UTF-8 searches -- see https://gerrit.vesnicky.cesnet.cz/r/82/ .
Dear Bug Submitter, This bug has been stagnant for a long time. Could you help us out and re-test if the bug is valid in the latest version? I am setting the status to NEEDSINFO pending your response, please change the Status back to REPORTED when you respond. Thank you for helping us make KDE software even better for everyone!
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Trojitá is no longer maintained, please switch to a maintained alternative like https://apps.kde.org/kmail2/ Sorry for the inconveniences.