The email server has the email. It can be retrieved unless it was purposefully searched for her emails and they were removed from it.
And that is exactly what was done
A good bit flipper could rebuild the mailbox. Most of the time whe email is “deleted” the only thing that happens is that pointers to the data are changed. The data is still there just gotta look hard enough.
No doubt they removed all the emails. Why not? If they turn them over the whole administration crashes down and maybe even an impeachment. If they delete them (or hide them) then it becomes a matter of “will” on the part of the repubs. They probably made the correct choice.
I manage Exchange servers. As soon as we’re notified that a person’s email is being subpoenaed for legal discover, that mailbox is placed on “legal hold”, and nothing in it can be deleted until the hold is removed.