Why is the computer sending spam to the server?

: Why is the computer sending spam to the server?
I have a server which is connected to three computers. For the past week, the server has been receiving spam messages from one of the computers. I have tried to scan all the three computers for viruses (of which i found some viruses) but still after removing them, the spam mails are still being received by the server.
Note: The server has also been scanned and no viruses were detected.
What could be the problem?

Answers and Views:

Answer by Laurence I
malware, this isnt seen as a virus so AV’s dont look for them

SPYBOTS&D or MALWAREBYTES should sort it out.

Answer by gobi nath
check the system settings disable sharing option or one system monitering aa another system without permission because some antivirus secure these type of process

Answer by Colinc
Do a FULL scan on ALL machines with the free scanner from https://malwarebytes.org
But it is highly likely that this is NOT from your machines but from outside. Security on mail servers is very difficult, one slightly incorrect setting and every machine in the world can pretend to be one of your machines. Also if the server also hosts web sites, or is showing any web host as a trusted server someone can easily have scripted a spam mail serving script onto the web host through an unfiltered injection URL. You do not tell us what type of server you are using so we can’t give any useful advice with any safety.

Answer by Ben
It is because your computer has so many viruses and malwares that want to use your Internet free of cost 🙂 to connect back to the creator

Use any smart anti-virus like Avira and other and a malwarebyte + Firewall. By doing this the malware would not be happy.