in the documentation we can read :
but i can't confirm is that true.
The message store is where all email messages and file attachments reside. Messages are stored in MIME format. A message that is sent to multiple recipients who have accounts on one mailbox server are stored only once in the file system.
by creating 2 accounts sans send then a mail we can see 2 files in the directory structure
we take the help from : Account mailbox database structure - Zimbra :: Wiki
thank's for reading me