20249 amavis    20   0  468104 317324  10760 S  0.0 31.2   0:19.76 clamd

Aha! There's your culprit. Clamd (a virus scanner) is using over 30% of your RAM. That's not the kind of program that you want to run in a resource-constrained environment such as a 1GB linode. Especially if you don't get a lot of email through that linode.

On the other hand, the weird mix of php-fpm and php-cgi makes me wonder whether it was appropriate to change the mpm_prefork settings. Your server seems to be set up in a rather unusual way, possibly because of the control panel you chose.

I tried to kill amavis with port number, Its not working. Can you please suggest me how to disable or best open source control panel ?