Looks like some spammer is using your mailserver to send his spam.
- Is the server up to date?
- Are you running any websites that have somekind of mail option and are they all up to date?
- Are your users email passwords strong enough?
- Did you scan your system for any rootkits, trojans, backdoors and local exploits?
Best thing todo now is to take the server offline and see how the spammer managed to abuse your system.
Good luck and have fun.
Never execute code written on a Friday or a Monday.