How to Secure Server from SPAM

Its more along the lines of, what can i do to my servers to help prevent any thirdparty from using it to send spam.

I have 6 servers, all with WHM/Cpanel

I use the servers for myself and not hosting so i know there are no scripts on there which a client might have installed which is being used to send spam.

Recently AOL blocked one of my servers because apparently it was sending out spam (according to them)

1) Is this possible ? When i am the only user and the only thing i have which sends out emails is a phpbb (registration activation emails)

2) What basic steps can i take to secure my server more, so that it can't be used to relay spam.

Any help would be appreciated

 

 

 

 

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