Mailboxes with this extension are file directories. These directories contain data_NNNN_ files, each file contains one or more messages, in the format similar to one used in Text Mailboxes.
The index.bdx file contains the mailbox index, and includes information for all messages stored in all data files.
As new messages are added to such a Mailbox, they are either appended to the existing data files, or stored in a newly created data file, depending on the message size and the size and number of messages stored in the existing data files.
When messages are removed, data files can be merged to keep the total number of data files low.