The authorities are unlikely to just seize the servers of such a notable business without warning. Even if they were to do so though, Lavabit and Levison have s

The authorities are unlikely to just seize the servers of such a notable business without warning. Even if they were to do so though, Lavabit and Levison have shown us that US mail providers can be compelled to cough up keys... and the nature of full disk encryption means doing so will inevitably enable access to bystanding client mail. It's understandable why FDE is used, and it's still slightly better than nothing, but per-mailbox encryption would allow for more selective disclosure in such extraordinary circumstances.