The mailbox format that stores each email as a separate file is Maildir.
Maildir represents a mailbox as a directory rather than as one large database or concatenated file. Each message receives a unique filename, and a typical Maildir contains three important subdirectories: tmp for messages being delivered, new for newly delivered messages, and cur for messages that have been seen or processed by a mail application.
This design was created by Daniel J. Bernstein around 1995, notably for the qmail mail system. Its major advantage is that delivery can use atomic filesystem operations instead of relying on a central mailbox lock. A reader should not see a partially written message because delivery completes the file in tmp before moving it into new.
The main alternative in the choices is mbox, which traditionally stores many messages sequentially in one file. EML describes an individual message file, while PST is Microsoft Outlook’s larger proprietary data-store format, not the name of this one-message-per-file mailbox architecture.