sudo mkdir -p /Library/Server/Mail/Data/rules/$USER_GUID
sudo chmod 775 /Library/Server/Mail/Data/rules/$USER_GUID
sudo nano /Library/Server/Mail/Data/rules/$USER_GUID/dovecot.sieve
(just type #replaceme for now, control+x, y, then return)
sudo chown -R _dovecot /Library/Server/Mail/Data/rules/$USER_GUID