Also, an enemy intelligence agency can greatly benefit from access to something like a mail server even if not a single classified or “sensitive” document is obtained.
Simple traffic analysis can reveal much. EX. Enemy country raises the alert level of it forces or moves forces towards a front. They then watch and see if their is any higher volume of traffic to determine if their move was detected. Even if the traffic was successfully encrypted the volume tells them something.
I read that agents would monitor such things as late night delivery of pizzas to the Whitehouse, to determine if the Whitehouse was in some kind of crisis mode.
My point is you do not need access to any classified documents to gain important intelligence from access to something like a mail server.