Email names hacked!

Hi, sorry if I've posted this in the wrong place but wasn't sure which topic to put it under.

Had my server running now for over 3 months and everything is fine until a few days ago when I started to receive emails from the server telling my it had blocked an email as it had an .exe file attached to it, which was great. This now happens on an hourly event but when I look at who they were sent to it's every mail account I have on my server, including mail forwarder's. Some how someone has been able to look at all the mail accounts and forwarder's on my server, how was this possible?

Any info would be great fully received,
Lee.

it would be best to check your server with experienced sysadmin or hosting suport as maybe some part of your server like account or root was hacked....