Finding source of outgoing email spam

[Xavier12]

Hey guys,

Hope you're well. A part from the email limit notification when a customer sends emails over the limit in outgoing mail, is there any other source that we need to find that is using our system to spam? Is there a hidden location that a system admin will have to seek to find an abusing account or cause for the spam that will not display in Plesk? Sendmail is disabled, so that isn't the issue.

We are using postfix and dovecot on Cloudlinux/Centos 6.7

Please advise as soon as possible. Thanks so much.