Stop the imail service and move the folder to the dive you would like them on then go into the registry and under each domain you want to move.
for x64 [HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Ipswitch\IMail\Domains\domain.tld]
for x86 [HKEY_LOCAL_MACHINE\SOFTWARE\Ipswitch\IMail\Domains\domain.tld]
in this reg file there is a line/entry
"TopDir"="(Current Drive):\\(current Folder)\\domain-folder"
edit this to change the directory for all users on that domain.
restart the service and test to verify everything is working properly.