we have a setup with (CentOS 6) Director+Dovecot, Maildir as storage on NetApp NFS v3. Every time I try to switch to NFS v4 I found issue with lock (and others). So for me NFSv4 with Maildir is "unstable" or need a fine tuning that I don't know.
I found the same thing, and turning off write delegation seemed to have solved the problem. I still don't know why, though. Joseph Tam <jtam.h...@gmail.com>
