Justin Cooper was an IT person hired by Clinton. When he turned up the new servers he would create a .pst or .ost file and copying the existing emails to it. He would then transfer those old emails onto the new server.
If you want to investigate go to the last twenty or so pages of the .pdf in the link below.
Sauce from Q's post is below.
https://www.judicialwatch.org/wp-content/uploads/2018/08/JW-v.-DOJ-02046-Hillary-R.-Clinton-Part-24-of-24.pdf