OK, now let me describe the problem:
The backup software wasnt able to write the data back to the backup server and failed. The software exited in bad state, causing all processes to go zoombie and a fatal exception in the kernel. This caused server to go high on load and we had to stop all services on server for 20+ minutes. Then regular mailservices were started except IMAP & webmail. After a reboot which took around 3-4 mins everything is fine now.
We apologize for the problems caused. The problem is being reported to the software writers and necessary precautions will be taken care of now on.