RHEL 5.7 huge load average

Hello,
After last kernel update I faced big load average, more than 220 for a server with 16 virtual CPU, 16GB RAM, 4 GB swap while just half of RAM is consumed and none swap.
Server mail role is sendmail and ps aux |egrep "D|Ds" shows more than 170 sendmail process. Normally server manage ~100000 emails/hour and before update, same amount of emails rised load to max. ~80.
Now ps alx show wchan as "-" so there is no clue related with what I/O are process waiting for ( i dont how it was before as i had no that problem)
Actual kernel version is kernel-2.6.18-371.8.1.el5.
I may add that no changes in configurations, mailqueue size or traffic occured.
Any advice is welcome.

Thank you