A quick look through the logs revealed this in /var/log/messages:
Nov ** **:**:** servername livesguardian(7032): Failed running maildirI had a look on Google and found no help. Trying to run the command "maildir" on the server resulted in a "no such file" error. Comparing directory listings of other servers, I noticed that /usr/bin/maildir didn't exist! I have no idea why, so decided to copy the maildir binary from the other server.
All is working swimingly now.