Need a lot of help

Folks. I originally posted this in the email section, but think it might be better suited in security. Folks. I'm very new so please bear with me My apologies if this has been asked before. My knowledge of WHM amounts to about 1 out of 100, as In the past i've had a hosting plan. However, the business is expanding, so i decided to buy a dedicated server. Maybe iv'e bitten off more than I can chew. I'm still trying to get my head around the multitude of different options in WHM. My first option is how to secure the Dovecote mail server. The dedicated server has been running 2 days and already i can see rogue sent emails in the delivery reports. Any ideas how this happened already ? Code: Event: success success User: useraccount Domain: usersdomain.com Sender: useraccount@server0-0-0-0-xxxxx.com Sent Time: Nov 19, 2014 9:15:21 PM Sender Host: localhost Sender IP: 127.0.0.1 Authentication: localuser Spam Score: Recipient: someone@gmail.com Delivered To: someone@gmail.com Delivery User: -remote- Delivery Domain: Router: lookuphost Transport: remote_smtp Out Time: Nov 19, 2014 9:15:21 PM ID: 1XrCah-00080m-DR Delivery Host: gmail-smtp-in.l.google.com Delivery IP: 64.233.166.27 Size: 643 bytes Result: Message accepted