A high performance mbox-replacement created by Mark Crispin while at 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.
- 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.
- At least on 32 bit systems, mailboxes are limited to 2GB size. (A mailbox can't be accessed anymore without manual fixing if it reached 2GB (minus 1B) of size on disk.) This is probably UW-IMAP -specific implementation problem rather than actual mailbox format issue.