lmtpd service unavailable errors

[Alex Cresswell]

I am seeing error messages such as:
Nov 16 05:45:31 imap sendmail[18058]: hAGBjVX18058: to=npimon, 
delay=00:00:00, x
delay=00:00:00, mailer=cyrus, pri=32496, relay=localhost [[UNIX: 
/var/imap/socke
t/lmtp]], dsn=5.6.0, stat=Service unavailable
in the maillog and random vacation messages seem not to show up. 

 From the config files:
/etc/cyrus.conf:  lmtpunix      cmd="lmtpd" 
listen="/var/imap/socket/lmtp" prefork=0
/etc/services:lmtp              2003/tcp

 From my .mc file:
MAILER_DEFINITIONS
Mcyrus,         P=[IPC], F=lsDFMnqA@/:|SmXz, E=\r\n,
                S=EnvFromL, R=EnvToL/HdrToL, T=DNS/RFC822/X-Unix,
                A=FILE /var/imap/socket/lmtp

The socket exists, and this problem doesn't occur for every message, 
just some select few.  Any ideas, or pointers where I can get more 
debugging info from?

-Alex