Forgot : I configured the compression with LZ4 algorithm (it saves a lot of space) and encryption of the mails. The encryption uses a master key. It is possible to encrypt every single mail file using the password of the account, but if the user forget or lose (and it happens sometime) his own password all the mails are gone ... Another option to explore is to autmatically encrypt every mail with the GPG public key of the owner, again if he lose the GPG private key or the password, the mails are lost forever, but unfortunately I don't see any interest in this by my clients.