How did the NethServer disk get partitioned? AFAIK NethServer "formats" the disks at installation!
It may work if you moved the mail folder over to home and create a symlink in the root partition...
Stop mail services first!
After moving and symlink, reboot.
It might be a better idea to backup the whole server - clean install of Nethserver (Letting Nethserver format the whole disk) and restoring!
My 2 cents