It doesn't really matter. Any method of server-side encryption for email (and many other) systems as we currently understand them has inherent flaws.
The simple fact is that key material is accessible in RAM, and even if it weren't, the data still must be decrypted at some point, and once the server is compromised, you can likely capture that decrypted data.
That's not to say you shouldn't do it -- it helps prevent accidents like unwiped drives getting out, and might be a reasonably effective obfuscation against some attacks, but it just isn't secure in the same way that real end-to-end encryption can be.
The simple fact is that key material is accessible in RAM, and even if it weren't, the data still must be decrypted at some point, and once the server is compromised, you can likely capture that decrypted data.
That's not to say you shouldn't do it -- it helps prevent accidents like unwiped drives getting out, and might be a reasonably effective obfuscation against some attacks, but it just isn't secure in the same way that real end-to-end encryption can be.