Linux Fans – kindly PLEASE assist me with identifying the spam source on my Centos6.9 smtp sendmail server

The server has also httpd with PHP services running
I did read the guide here :
I did install EXIM

My client complains they never had this happen on the Windows Server 2000, 2003, 2012 …… (yes i know Linux is better)
here are a list of commands that I have ran

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Did read and try:

Linux Malware Detect

Tracing Spam from PHP Scripts