Our client mail server got flaky between around 11AM and 1PM today.
I am pretty sure the cause is brute force password attacks exhausting some kernel resource but I have not been able to identify the resource being exhausted.
The reason I believe this is the cause is that in the last two days the number of IP addresses we lock out for these sorts of attacks has increased from a typical number of several hundred to over 15,000. This is probably the result of a new Windows virus that is allowing the creation of huge botnets. This is something we see periodically.
I rebooted the server which restored it to normal functionality and will continue to try to determine what is being exhausted and correct it.