Mail is stored in an outgoing queue awaiting transfer to a remote server or in a local mail store accessible by local mail users.
By default, outgoing mail messages are stored in the following spool directory on the startup disk in /Library/Server/Mail/Data/spool/.
This location is temporary, and the mail is stored until it’s transferred to the Internet. These locations can be moved to any accessible volume if you create a symlink link to the new location.
Mail service stores each message as a separate file in a mail folder for each user. Incoming mail is stored on the startup disk in /Library/Server/Mail/Data/mail/.
You can change the location of mail folders and indexes to another folder, disk, or disk partition. You can even specify a shared volume on another server as the location of the mail folder, although using a shared volume negatively affects performance.
For remotely mounted file systems, NFS isn’t recommended. The incoming mail remains on the server until deleted by a mail user agent (MUA).