Dump a user's mailbox to a file (e.g., .mbox) from server for archival
I ended up doing this via Terminal on the server.

cd /Library/Server/Mail/Data/mail
sudo ls -l users | grep &lt;username&gt;


(where  is the username you're looking for)

This returns the user's ID (e.g., 006638A6-9664-4CB4-8814-525A253B6E33)

sudo cp -a &lt;id&gt;/ ~/Desktop/&lt;username&gt;


This will copy the user's mail directory to a directory on the desktop. I then dumped that directory onto an external hard drive used for archiving former employees' mailboxes.

Thankfully we'll be switching away from macOS Server (which will be removing the Mail service in an upcoming update) to Exchange, so I won't have to deal with this anymore!