Dovecot won't deliver the first mail to a mailstore on nfs share

Dovecot won't deliver the first mail to a mailstore on nfs share

Good morning, I'm building a mail platform using
dovecot/postfix/mysql/iredmail and stuff. The business end is on ubuntu
13.04, the mailstore is an nfs share on ubuntu 12.04.
Now when I try to send a mail to a new account, I get this in the postfix
logs.
Aug 29 12:39:23 core-01-mail postfix/pipe[8374]: 5BEFF7A1: to=,
relay=dovecot, delay=1009, delays=706/0.37/0/302, dsn=4.3.0,
status=deferred (temporary failure)
however, if I tell it to use a local directory as a mailstore, it works
fine. So far it looks like my NFS share is busted, except....
If I then change it back to using the NFS mailstore, it continues to work
fine. It's only the first mail that isn't working, wha?
I've turned on full verbose debug logging for dovecot, but I'm not seeing
any errors or warnings.
incidentally, here's my nfs mount
172.1.1.15:/voldata /data nfs nfsvers=3,hard,intr,auto 0 0
any ideas?