When I read:
"The hacker compromised the firm’s global email server through an “administrator’s account” that, in theory, gave them privileged, unrestricted “access to all areas”. The account required only a single password and did not have “two-step“ verification, sources said."
Then it is nothing technical, but a security policy issue. A typical the chain is as strong as the weakest link....
That an email administrator account can have access to the whole network and servers is also a security policy flaw. Bad design and bad practice IMHO.
Hence SME Server has a seperate admin account next to the root account and the user panel contrib...