Note about Email in Tier 3 -- it's logically placed in Tier 3, but actually has the same security mechanism as Tier 1. The reason it's placed in Tier 3 is that

Note about Email in Tier 3 -- it's logically placed in Tier 3, but actually has the same security mechanism as Tier 1. The reason it's placed in Tier 3 is that if it's compromised, it at least doesn't directly compromise Tier 1.