When an e-Mail which has been read, is moved manually to another KMail2 Folder, the associated e-Mail file is placed the "moved-to-Folder" sub-directory "new" and not as expected in the sub-directory "cur".

The following CLI "find" commands search for the e-Mail files:

 > find .local/share/local-mail/ -type d -name 'new' -exec ls '{}' \;
or
 > find .local/share/.local-mail.directory/ -type d -name 'new' -exec ls '{}' \;

To find which KMail2 Folder contains the "new" sub-directory containing the moved e-Mail(s) use:

 > find .local/share/local-mail/ -type d -name 'new' -print -exec ls '{}' \; | less
or
 > find .local/share/.local-mail.directory/ -type d -name 'new' -print -exec ls '{}' \; | less

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Affected e-Mails have to be selected from within KMail2 and the "moved to the 'cur' sub-directory" with <Ctrl-U> and then <Ctrl-R> -- "mark as unread" and then "mark as read".