KMail can save messages in the following formats:
This traditional UNIX format (the oldest) saves all messages in a single file. Directories are also simulated by means of individual files. The individual mails can only be distinguished by the From line.
In contrast to the mbox format, the MailDir format has a unique file name for every message. Messages are stored in directories and can easily be identified and further processed. KMail uses the MailDir format (as the more convenient alternative) by default, but MailDir is not supported by many other e-mail programs. This can be a disadvantage if you sometimes want to use a different e-mail program. However, there is no problem processing old e-mails in mbox format when using MailDir. Just copy or move the mbox file into a MailDir folder.