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I would like to propose this idea:
As we type a subject in kmail, it will be stored as for example the recent email addresses.
The next time that we start typing the subject of a new mail, if we type the same letters as a subject stored in the recently typed subjects, it will be proposed in a list.
Same behavior as the recently typed addresses. Also, it will have an option to enable/disable it and an option to delete the list with the stored subjects.