Mailbox encryption is near pointless since at the least it needs to be encrypted at both ends not to mention relays.
For email each individual message should be encrypted if you want any confidentiality and even then the meta data is in the clear.
And this is because in order to send or receive an email the provider needs to access it. If they put it into a box later on to which they do not hold the key that is just security theater at that point.
