Concurrency limit warning in Postfix leads to server lock

[Robert Fitzpatrick]

I have dilemma with one of our 5.4 server mail gateways. About 2-3 times
a month now the server SMTP and related services stop responding. I find
myself not able to login, just sits there after entering user name. I
have to reset the server and the only thing I can find with an 'egrep
(fatal|error|warn)' in the messages and maillog are these concurrency
limit warnings minutes before the issue started...

Sep 15 07:19:02 esmtp postfix/smtpd[2789]: warning: Connection
concurrency limit exceeded: 51 from unknown[88.238.96.247] for service
smtp

This seems to be an attacker of some sort, I block them and the issue
goes away, of course. I posted my issue to the Postfix list, but was
told this should not be taking down my server and to find out why I'm
not able to login when this happens. I am looking for help on where to
look to determine this, can someone give some guidance? Some other log I
should examine? The only thing I can spot that looks possibly out of
place is nfsd running at 6-8% CPU. I do a backup from one other server
to this server via nfs. I checked and all that backup was finished
couple of hours prior to this latest issue, but the nfsd process seems
to be taking more CPU than normal. And when I reboot, the nfs connection
I have in /etc/fstab takes several seconds to initialize.

-- 
Robert