Setting lmtp_user_concurrency_limit causes anvil permission error

[Tom Sommer]

I've set lmtp_user_concurrency_limit to 5 and now LMTP throws this at me 
for every delivery:

Apr 07 13:38:33 lmtp(4434): Error: 
net_connect_unix(/var/run/dovecot/anvil) failed: Permission denied

ls -l /var/run/dovecot/anvil
srw------- 1 root root 0 Apr  7 13:32 /var/run/dovecot/anvil

If I set lmtp_user_concurrency_limit to 0, the error goes away.

-- 
Tom