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. The messages are stored in the respective 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 want to switch to a different e-mail program later and still access your old e-mails. Processing old mails in mbox format is no problem, however, if you select MailDir. Just copy or move the files to the MailDir folder.