OS X Server MailBox size expand
You two options to change the mail box location:

You can either change the Server Data location to an external volume or a share:
Open Server.app. In the left pane choose Server -&gt; server_name -&gt; Settings -&gt; Service Data. Hit the button Change Location and choose another volume. This will copy all service data to a new location. Before migrating the services will be shut down automatically.

Or you copy the Mail folder /Library/Server/Mail to a different location
Stop the mail service. Copy/rsync the folder /Library/Server/Mail to a different volume/share. Keep the original permissions! Remove the original folder and symlink back the new location to /Library/Server/Mail.


After moving the mail data location adjust the mail box size and start the mail service again.