mbx
A high performance mbox-replacement created by Marc Crispin of University of Washington for UW-IMAP. Each message in the file is preceded by a record carrying all the metadata that IMAP-protocol needs. This allows changing the metadata easily by modifying the fixed-size header, rather than moving data around in a file like with mbox.
File locking is handled more intelligently (only appends may need to wait for locks), making this format a good choice for shared mailboxes.
Downsides contain:
- Messages don't get deleted from disk until there's only one client accessing the mailbox, so you may need to add some forced downtime to get it done.
- Works only in local filesystem.
- Expunging is still as costly and as fragile as with mbox.