Claws Mail uses the MH format according to its manual.
Mbox vs Maildir: Mail Storage Formats states that
The Unix world has two ways of storing mail messages, the traditional mbox format and the newer maildir format. Postfix and Dovecot supports the two mail storage format so you can use any format, but I highly recommend you use the maildir format.
Is the MH format used by Claws Mail different from both Mbox and Maildir?
Is a special designed format?